In baseball, a tag out, sometimes just called a tag, is a play in which a baserunner is out

Out (baseball)

In baseball, an out occurs when the defensive, or fielding, team effects any of a number of different events, and the umpire rules a batter or baserunner out. When a player is called out, he is said to be retired...

 because he is touched by the fielder's hand or glove holding a live ball

Live ball (baseball)

In baseball, when the ball is alive , the game can proceed. The pitcher may pitch the ball , the batter may attempt to hit such a pitch, baserunners may attempt to advance at their own risk, and the defense may attempt to put the batter or baserunners out.-Definition:The ball becomes live when the...

 while the runner is in jeopardy. A baserunner is in jeopardy when:

1. he is not touching a base (other than after overrunning first base or when advancing to an awarded base, such as on a base on balls
   Base on balls
   A base on balls is credited to a batter and against a pitcher in baseball statistics when a batter receives four pitches that the umpire calls balls. It is better known as a walk. The base on balls is defined in Section 2.00 of baseball's Official Rules, and further detail is given in 6.08...
   
   );
2. he is touching a base he has been forced to vacate because the batter became a baserunner (a forced runner
   Force play
   In baseball, a force is a situation when a baserunner is compelled to vacate his time-of-pitch base—and thus try to advance to the next base—because the batter became a runner. A runner at first base is always forced to attempt to advance to second base when the batter becomes a runner...
   
   )
3. he has not tagged up
   Tag up
   In baseball, to tag up is to retouch or remain on the runner's time-of-pitch base until the ball either lands in fair territory or is first touched by a fielder. A runner must, by rule, tag up only when a batted ball is caught by a fielder . After a legal tag up, even if the ball was caught in...
   
    on a caught fly ball;
4. he failed to touch a base when he last passed it, or to touch them in order; or
5. he is touching a base that a preceding baserunner is also touching, except when forced to vacate the previous base because the batter became a baserunner

A tag is the most common way to retire baserunners who are not in danger of being forced out

Force play

In baseball, a force is a situation when a baserunner is compelled to vacate his time-of-pitch base—and thus try to advance to the next base—because the batter became a runner. A runner at first base is always forced to attempt to advance to second base when the batter becomes a runner...

 (as in (1) above), but a tag can put runners out on a force or a non-force play, as well as on an appeal play

Appeal play

In baseball, an appeal play occurs when a member of the defensive team calls the attention of an umpire to an infraction which he would otherwise ignore.-Appeal Play Situations:A runner shall be called out, after a successful live ball appeal, if he:...

 (as in (3) or (4) above). Runners attempting to advance are sometimes thrown out, which means that a fielder throws the ball to someone covering the base, who then tags the runner before the runner touches the base. A runner who leads off a base too far might be picked off

Pickoff

In baseball, a pickoff is an act by a pitcher or a catcher, throwing a live ball to a fielder so that the fielder can tag out a baserunner who is either leading off or about to begin stealing the next base....

; that is, the pitcher throws to a fielder covering the base, who then tags the runner out.

Attempted tag outs sometimes can instigate fights in baseball. Often the infielder will tag the baserunner too hard, or the baserunner will slide into base with enough speed and force to knock over the infielder attempting to tag out.