Definition of TAG
A player who is called a TAG is a "tight aggressive" player. The tight player will play few hands, but very aggressively. The TAG style is the recommended style for beginners. It is also a winning style when moving up in limits. If you are playing full ring tables (9 or 10 players), the TAG style is even more recommended since you have to play tighter in the full ring. If you are playing heads-up poker, however, this style is clearly not optimal.
What does a TAG style look like?
If you are playing 6 player cash games (shorthanded) and you have a HUD (Heads-up display that your tracker like PokerTracker 4 will display on your tables), you will notice that the TAG player will have stats that look like this: VPIP between 17% and 21%, preflop raise (PFR) between 12% and 19%, 3bet % between 5% and 12%. His overall aggression factor will be between 2 and 4. Then you know you are dealing with a TAG player. If his VPIP is higher and he is close to 35%, then he is considered a LAG (loose aggressive).