Tagging is a means of putting one or members names in a post to get their attention by notifying them in the forum that they have been mentioned (tagged) and/or send them an email - providing the member/s have the appropriate Notification settings (see image below) in their profile:
To Tag a person you type a @ character, immediately followed by the member's username NOT their forum name (unless they are both the same).
For instance, if I want to get member Gilly's attention I type
@paul which becomes Gilly - paul is Gilly's username - Gilly is Paul's forum name.
To find out what to use, simply place your cursor over someone's name - either in the Users Online in the Last 24 Hours list or anywhere where their name appears in the forum - the image below shows your cursor over Gilly's name in the Users Online in the Last 24 Hours list: