Regenerate is a shield you set up ahead of time. When you pay a regenerate cost, the creature gains a regeneration effect until end of turn. The next time that creature would be destroyed that turn, it isn’t: instead you tap it, remove it from combat, and heal all damage on it. The reminder text on cards spells this out exactly. It only stops destruction, so it won’t help if the creature is sacrificed or its toughness drops to zero.
In play, regenerate lets you attack or block more aggressively. A creature can survive lethal combat damage or a destroy spell, then come back tapped and out of combat. That tempo loss is real, but keeping a key creature alive is often worth it. Regenerate costs usually require mana, so holding some open can make opponents hesitate.
