The "Deferral" for the 401k means that you're not collecting your pay immediately, but instead diverting it to a retirement account (Roth 401k in this case).
This article defines deferral well:
What is the difference between a regular 401(k) deferral (pre-tax) and a Roth 401(k) deferral?
Under either a regular 401(k) deferral or a Roth 401(k) deferral, you
make a deferral contribution by electing to set aside part of your
pay (by either a certain percentage or a certain dollar amount). For
a regular 401(k) deferral, the taxable wages on your W-2 are reduced
by the deferral contribution; therefore, you pay less current income
tax. However, you will eventually pay tax on these contributions and
earnings when the plan distributes the regular 401(k) deferrals and
earnings to you. The result is that the tax on the regular 401(k)
deferrals and earnings is only postponed.
A Roth 401(k) deferral is an after-tax contribution, which means you
must pay current income tax on the deferral. Since you have already
paid tax on the deferral, you won’t pay tax on it again when you
receive a distribution of your Roth 401(k) deferral. In addition, if
you satisfy cer tain distribution conditions, then you won’t have to
pay tax on the earnings either. This means that the distribution of
the Roth 401(k) earnings can be tax free not just tax postponed.
Traditionally, this deferred compensation typically was directed to a 401k, but now that Roth 401k is another available option, deferred compensation can be directed there as well.
