FizzyOrange got it right. "Screen grab" is nicely asking a graphical system (X11/xorg-server or a Wayland compositor or whatever) via an api to give you an image of either the whole desktop or some particular rectangular part of it. And you can do it 30 times every second (or more) to get a video. OBS uses such APIs to get video from the screen for saving to a file or streaming to Twitch or whatever. Various tools can be used to get screenshots and save them to files. Etc.

Heck. On my work machine, because they require us to meticulously log the time we spend on individual tasks, I've got a script running that uses ImageMagick's import command to grab screenshots of my desktop and save them to files once every 5 minutes so I can refer back to them while logging my time.

And as FizzyOrange said, various Wayland compositors have workarounds for the fact that there isn't (or rather wasn't until recently) a way to do screen grab in a standard way that would work across all compositors which properly and comprehensively implement the Wayland protocol. I use Sway on my personal machines and it's based on something called wlroots which has built-in a nonstandard extension to the Wayland protocol that allows screen grab. But once wlroots adopts the new standard way of doing screen grabbing, the nonstandard extension will be unneeded/obsolete.