I was perusing Reddit's r/languagelearning, and this comment by u/whosdamike piqued my curiosity (especially the parts I highlighted in bold):
Here is an update I made about learning Thai via pure comprehensible input. I use learner-aimed videos that gradually ramp up in difficulty. Beginner videos used visual aids to communicate meaning alongside the spoken speech; these visual aids mostly dropped by the lower intermediate level.
At the beginning, 100% of my understanding was coming from pictures/drawings/gestures/facial expressions. Now almost all my understanding comes from the spoken speech and explanations for new words are almost entirely explained in Thai.
This caught me by surprise, because obviously explanations in one's target language should count as comprehensible input. But then...
Question: What input doesn't count as "comprehensible input"?
If explanations count as "comprehensible input", then when a teacher teaches vocabulary and grammar (assuming they're using the target language when teaching), that input is comprehensible too. It seems studying textbooks, example sentences, and looking up words in a dictionary are also ways to get input that is comprehensible. Besides output, I'm struggling to conceive of plausible language-learning activities which would not be considered comprehensible input.
Perhaps "comprehensible input" simply means "input which is not incomprehensible", but that doesn't seem like particularly profound. There must be more to this story.