1. As a synonym for "off-screen". For example, on >>2676087
2. For images with some kind of border, and a character or object extends beyond that border—either partially, like the frame is some kind of window or door the character's passing through (like >>2430402) or entirely, like the character is floating in front of a small square of background (like >>2685711).
From a skim over pictures tagged with [out of frame,](/tags/out+of+frame) it looks like meaning #2 is the much more common one. BUT there's a related tag [head out of frame](/tags/head+out+of+frame) which consists entirely of pictures where a character's head is off-screen. So as long as `head out of frame` exists, it's going to make people think `out of frame` is supposed to be about meaning #1 instead.
I'm not sure how to untangle this.