Technically regenerate is not a keyword mechanic, it is a keyword action. It is a set of instructions that players apply to the game when a regeneration effect is applied.

When a resolving spell or ability creates a regeneration effect, it is creating a replacement effect. This replacement effect does four different things. It means that the next time that the appropriate permanent would be destroyed that instead it 1) remains in play, 2) becomes tapped, 3) has all damage removed from it and 4) if it is in combat remove it from combat.

Activating an ability or casting a spell that generates a regeneration ‘shield’ is not actually regenerating. A creature only actually regenerates when this replacement effect is applied. This can matter as some abilities care about when a permanent actually regenerates.