Pressing pushes the last of the coffee through.
The plunger forces the brew through the paper. Pressing slowly avoids squeezing bitter fines into the cup; the hiss means only air is left.
What it is
In an AeroPress, the plunger pushes the brew through the paper filter.
Why it matters
Gentle pressure finishes the brew quickly and cleanly. Pressing hard forces fine particles and bitter compounds into the cup.
In practice
Press slowly and steadily, for about 20–30 seconds. Stop at the hiss: that's air, and what's left is mostly bitter.