What does it mean?
I have a Cleffa with Psychic.
In the past there have been changes made regarding moves. One of the changes was the fact that some Pokémon could no longer learn moves they could learn previously.
For example, when the game launched Gyarados could learn dragon breath which later was removed from his move pool.
Snorlax could learn BS until the recent update but that is a legacy move for him now as well.
Psychic on cleffa might be a legacy move or not. This is not too relevant since you don't use cleffa in battle
A legacy move is a move that can no longer be obtained on a newly caught or evolved pokemon, or be hatched from and egg that was received after the move changes.
Some of the legacy movesets are better than the currently available moves, but just because a move is legacy doesn't necessarily mean it is more desirable. Mud Shot is a legacy move for Poliwrath, but my high IV Mud Shot/Ice Punch Poliwrath is still pretty worthless unless we can ever someday get the option for rerolling movesets.
