A Tag is a way to switch characters in Street Fighter X Tekken.
By pressing medium punch and medium kick at the same time with no joystick inputs a switch is initiated. The current character does a pose fitting for their character then they run out and their partner comes in. The pose is a dead giveaway to the opponent. The switching character or incoming character usually give a phrase that depends on how much health the player characters have left or the remaining health of the opponent.
This is the only "free" way to switch characters in Street Fighter X Tekken. The character that initiates the tag becomes invincible while posing but the partner is vulnerable the moment they appear on screen. The character that comes in has a very long "stop" animation from their running that makes this a poor choice to simply be used at will. Some players use all defense gems simply to greatly minimize damage to the incoming character. Fortify gems can eliminate most if not all incoming damage off a tag in the middle of a battle.
Cross Cancels usually (but not always) provide a free tag. Some other attacks such as Ryu's EX Shoryuken also provide free tags. Against some Zoning opponents a player can tag to take minimal damage by a Projectile. High HP characters like Hugo are usually tagged in, especially against characters that do little damage such as Poison. Low health characters such as the 850 HP Akuma or characters with 900 HP such as Ibuki and Ling Xiaoyu are not recommended for raw tag incoming as they can not take as much damage compared to other characters.