Migospel can increase the HP of a monster by 3, including enemies. This makes Migospel the universal version of the Kindness exclusive Force of Nature. Keep in mind that a damaged monster stays damaged, making it still vulnerable to cards like Asgore.
Like every other Magic ability, you cannot choose when to add the 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 stats (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.