Migospel can increase the maximum HP of a monster by 3, including enemies. This makes Migospel the universal version of the Kindness exclusive Force of Nature.
If the monster is damaged, it will heal it for 3 HP and still increase its maximum HP (but this in turn doesn't fully heal them, making them still vulnerable to cards like Asgore).
Like every other Magic ability, you cannot choose when to add the 3 HP and must use the ability immediately after placing the card. so it is best to have at least one ally card on the field when you do this, as it also cannot give the +3 HP to itself, again, just like every other Magic ability.
It is best used on taunts and other monsters with decent hp or attack (adding it to Ice Cap for example might not be the best idea as it will probably still die anyway), however adding HP to a monster in the early game can be advantageous. Especially useful in Kindness decks.