If you've moved the cursor to a particular line towards the top or bottom of
the screen, you may want to center the screen on that line. This can be
achieved by hitting zz. It is a quick way to center the cursor and ensure
that you have an equal amount of context on either side of the line of
interest.
Take care to not mistake this for ZZ which will save and quit the buffer.
See :h zz and :h z. for more details.
h/t Chris Erin