Share is not an existing ability within the Pokémon game series. It appears you might be referring to an invented or third-party concept, or potentially confusing it with similar mechanics such as the Exp. Share item.
However, if we are to imagine an ability named "Share," we can derive a fictional concept:
Share is a hypothetical ability that allows a Pokémon to distribute a portion of the experience points it earns in battle to an ally, even if the ally is not directly involved in the fight. This ability would emphasize team growth, allowing weaker Pokémon to gain levels and become stronger more rapidly.