Feast is a Kindness deck specific effect card. It works as an all ally version of Force of Nature, giving +3 HP to all your monsters, while also healing them by that amount. However, if it's used on a monster with 3 HP or less, that monster will get +4 HP instead.
This is useful in any situation, but best used if your whole board is full to maximize its effect. A good way to use this is with Moldsmal, Moldbygg and Moldessa. Due to Moldsmal and Moldessa having only 2 HP, this card will bring that up to 6, equal to base Moldbygg (who will end up at 9 thanks to Feast). And Moldbygg's ability will give +1 HP to itself as well as any other Mold monster at the end of your turn, making all of them hard to kill.