We sometimes manage to give the Runner a tag, typically by means of a trace. A tag is a clue to the Runner's identity or to where he or she is interfacing with the Net: it could be a description of the Runner or the Runner's favorite personalized Net icon; it could be the Runner's address, or perhaps the Runner's mother's address. The more tags we get on the Runner, the more we know about him or her.
We have many cards that can only be used, or are more effective, if the Runner is tagged. Also, while the Runner is tagged, we can take an action to pay to trash any resource card the Runner has in play.
If the Corp gives you a tag, it has gotten some information about your identity and/or whereabouts: a tag could be a description of you, either on or off the Net; it could be your address, or perhaps your lover's address. You can remove a tag by taking an action to pay .