Upon having a spell cast at you from a truly hostile enemy (DM discretion), you may choose to use your reaction to let the spell strike you with all of its force. Doing this forgoes trying to avoid or save from the spell for that round and the spell automatically hits or succeeds. The spell may still crit, if a crit is rolled.

Roll a 1d20. This roll cannot be altered. Things such as feats, abilities, or class features have no effect.

On a 20, you learn the spell regardless of requirements (Wizard, Druid Warlock, etc.) and the spell cast on you is nullified and you cast it back on the caster automatically succeeding or hitting them.
On a 19-2, you fail to understand the magic and take 5 extra damage of the spell's type. If no damage type exists take 5 force damage before the spell's effects take place.
On a 1 you fail to understand the magic and if the spell dealt damage, you take triple the damage. Also if the spell affects you for a duration, you are under its effects for 3x as long unless its duration is concentration based.

If you succeed on learning the spell, the spell is always prepared and does not count against the amount of prepared spells a you can have.
The casting requirements must still be met in order to cast it. (Level, components, spell slots, etc.)