Snagging is going over a snag-list. So generally rectifying problems.When a house is built, at various stages during that process you'll either get the site manager or customer go around the house and make a list of things that need to be sorted out. So perhaps a tile coming loose or something installed in the wrong place has been moved to the right place and it's left some repair work to do, that sort of thing.
You want to make sure you're only doing things within your capabilities though, so don't let them start trying to make you fix architrave that a tiler has cut too much off and whatnot, if it is you don't normally work with architrave (that's a rubbish example - but I couldn't think of another one quickly. haha).