Are Fate points involved? How is an aspect declared/invoked? The text says a skill check is involved.
Tagging is described in the SRD.
It means to make use of an aspect that isn't associated with your character (e.g. an aspect on another character, or on the scene).
All the normal rules of aspect use apply.
Using an aspect costs a fate point (with the exception that if you have invested some effort to create or introduce the aspect, you can tag it once, for free, if you do so immediately).
If you do it to influence a skill check then it can give you a +2 bonus or reroll). Otherwise you can use it for effect.
Also, if you tag an aspect on another character, they get the spent fate point.