A lump of royal jelly is a special comestible that is only found in beehives (it can also be created by a horn of plenty). Eating a lump of royal jelly will increase your strength (if not already maximized) by +1. If your legs are injured they will be healed. Your Hit Points are increased by 0-20 (or decreased if the jelly is cursed). If this would raise your HP above your max HP, there is a 6.25% chance (1 in 16) to increase your max HP by one.
Royal jelly is a popular comestible to store for the Ascension run, as it has a very good nutrition-to-weight ratio and can be eaten in one turn, both of which can be crucial concerns in the late game.