The user restores its own HP by up to half of its max HP.
Soft-Boiled restores the user's HP by up to the amount of HP that makes up half of its max HP. It will fail if the user's HP is full.
Outside of battle
Soft-Boiled can be used to restore the HP of another party Pokémon. However, the mechanics behind it is different, as it involves the user having to transfer its own HP in order to restore a target's. The user will transfer up to 20% of its own HP to the target. It will not work if the user has 20% or less of their max HP left.