"Revive" is an informal term to describe Special Summoning a monster from the Graveyard. The ability to return a monster from the Graveyard to the field is a relatively common yet nonetheless important card effect since it helps a player maintain field advantage and can, in some cases, allow them to bring back a monster that was difficult to Summon in the first place. Most of the more general-use cards that deal with this, such as "Premature Burial", are Limited or Forbidden, though there are much fewer legal restrictions on cards that deal with only specific kinds of monsters, such as "Infernity Launcher".
Sometimes it is easier to dump a Monster Card into the Graveyard before it has been Summoned and then revive it onto the field using one of the many revival cards available; to restrict this, many very powerful monster cards do not allow this, in particular nomi, Special Summon-only, Fusion, Ritual, Synchro, and Xyz Monsters.