Cold pulling is where you heat the hot end up to just past the melting point of your filament and push a bit of filament through it by hand until an inch or so comes through, then you cut the heat and let the nozzle cool down a bit, then pull the filament back through the extruder side.

This basically traps all the little bits of semi-melted plastic left behind from previous prints, and pulls them out as one big glob.

Different guides will give different advice about the temp to cool to, but basically you want it cool enough that it puts up meaningful resistance pulling the filament back out by hand but isn’t impossible.

You should probably do this on some routine, but at the very least after any clog and when you get print errors that you can’t immediately diagnose. Partial clogs are responsible for way more issues than you might think.