Pretty much the title, I'm pretty confused and the blender documentation doesn't help much : "Round the input value to down to the nearest integer multiple of Increment"
It does what it says on the tin.. :) .. it winds up being a staircase function:
.. with the rise and tread both being the 'Increment'. Here, shown with increments of 2, 1, and 2/3.
For any input X, the output will be X, rounded down to the nearest whole-number multiple of 'Increment'.
Thanks again to @Rich Sedman for this way of visualising functions..
Because of the great geometry nodes you can also watch easily the snap values with this node setup:
Just change the B-values and/or the x/y offset values and watch what happens.
As @batFINGER points out, in Python, it could be:
increment * (x // increment)