The poor in the passage are the have-nots that lived in the land. Or, like you said, the lower-class of the society.
Jeremiah as the prophet foreseeing the judgement and distraction of the Kingdom of Judah, and its capital, makes the distinction between the educated rich and the poor. The exploitation of the impoverished by the elites was a blunt violation of the law. Together with the open idolatry, it was the main reason that led to the destruction and the Babylonian exile.
The rich and powerful were the ones to blame, because unlike the illiterate poor, they had the access to the law and could read it themselves. But instead of keeping it, they violated it by breaking the commandments and rewriting their meaning.
8 “ ‘How can you say, “We are wise, for we have the law of the Lord,”
when actually the lying pen of the scribes has handled it falsely? 9
The wise will be put to shame; they will be dismayed and trapped.
Since they have rejected the word of the Lord, what kind of wisdom do
they have? 10 Therefore I will give their wives to other men and their
fields to new owners. From the least to the greatest, all are greedy
for gain; prophets and priests alike, all practice deceit. 11 They
dress the wound of my people as though it were not serious. “Peace,
peace,” they say, when there is no peace. 12 Are they ashamed of their
detestable conduct? No, they have no shame at all; they do not even
know how to blush. So they will fall among the fallen; they will be
brought down when they are punished, says the Lord. (Jeremiah 8)
According to the law of Moses, the poor and the needy had a special place in the Israelite society. As brothers, they should have been helped and cared for. They were not.
9 “ ‘When you reap the harvest of your land, do not reap to the very
edges of your field or gather the gleanings of your harvest. 10 Do not
go over your vineyard a second time or pick up the grapes that have
fallen. Leave them for the poor and the foreigner. I am the Lord your
God. (Leviticus 19)
10 “For six years you are to sow your fields and harvest the crops, 11
but during the seventh year let the land lie unplowed and unused. Then
the poor among your people may get food from it, and the wild animals
may eat what is left. Do the same with your vineyard and your olive
grove. (Exodus 23)
Unfortunately, the sabbatical years were not kept. The land was not respected and treated as God's property. As a result, the poor could not get the food they needed.
The people neglected God's warnings from the past and paid for it dearly:
27 “ ‘If in spite of this you still do not listen to me but continue
to be hostile toward me, 28 then in my anger I will be hostile toward
you, and I myself will punish you for your sins seven times over. 29
You will eat the flesh of your sons and the flesh of your daughters.
30 I will destroy your high places, cut down your incense altars and
pile your dead bodies on the lifeless forms of your idols, and I will
abhor you. 31 I will turn your cities into ruins and lay waste your
sanctuaries, and I will take no delight in the pleasing aroma of your
offerings. 32 I myself will lay waste the land, so that your enemies
who live there will be appalled. 33 I will scatter you among the
nations and will draw out my sword and pursue you. Your land will be
laid waste, and your cities will lie in ruins. 34 Then the land will
enjoy its sabbath years all the time that it lies desolate and you are
in the country of your enemies; then the land will rest and enjoy its
sabbaths. 35 All the time that it lies desolate, the land will have
the rest it did not have during the sabbaths you lived in it.
(Leviticus 26)
15 The Lord, the God of their ancestors, sent word to them through his
messengers again and again, because he had pity on his people and on
his dwelling place. 16 But they mocked God’s messengers, despised his
words and scoffed at his prophets until the wrath of the Lord was
aroused against his people and there was no remedy. 17 He brought up
against them the king of the Babylonians, who killed their young men
with the sword in the sanctuary, and did not spare young men or young
women, the elderly or the infirm. God gave them all into the hands of
Nebuchadnezzar. 18 He carried to Babylon all the articles from the
temple of God, both large and small, and the treasures of the Lord’s
temple and the treasures of the king and his officials. 19 They set
fire to God’s temple and broke down the wall of Jerusalem; they burned
all the palaces and destroyed everything of value there. 20 He carried
into exile to Babylon the remnant, who escaped from the sword, and
they became servants to him and his successors until the kingdom of
Persia came to power. 21 The land enjoyed its sabbath rests; all the
time of its desolation it rested, until the seventy years were
completed in fulfillment of the word of the Lord spoken by Jeremiah.
(2 Chronicles 36)
And what about the poor? What happened to them? God compensated them through the same guy that was the instrument of His punishment:
8 The Babylonians set fire to the royal palace and the houses of the
people and broke down the walls of Jerusalem. 9 Nebuzaradan commander
of the imperial guard carried into exile to Babylon the people who
remained in the city, along with those who had gone over to him, and
the rest of the people. 10 But Nebuzaradan the commander of the guard
left behind in the land of Judah some of the poor people, who owned
nothing; and at that time he gave them vineyards and fields. (Jeremiah
39)
