A meatball is a type of food created by zapping a spell of stone to flesh at rocks or gems (glass will not work). Meatballs have very little nutritional value, but they can be used to tame cats and dogs, and act like miniature tripe rations for the purpose of training them to steal from shops.
Meatballs have monetary value, so you can cast stone to flesh on worthless rocks and then sell the resulting meatballs for cash. This may not be the best option, because you could also then polypile the meatballs, and from your 100 meatballs get say, 100 lembas wafers, which you could sell for more than you would get for the meatballs. This however has a risk of creating a flesh golem.
If you have a method of taming, polypiling meatballs is an almost win-win scenario for lower levels (unless you get tins). If you get a flesh golem, the golem is unlikely to resist taming, and is a useful pet to have for lower levels. Hit it with a footrice corpse to transform it into a level 13 stone golem. If you get food, you can do whatever you want with it, although if you get fruit, vegetables or herbs, you may want to save some for training and/or taming non-carnivorous pets.
With luck and a little polypiling, one can obtain cockatrice eggs, tins of spinach and tengu meat, and other rare comestibles. However, creating such comestibles reliably requires great numbers of meatballs. Thankfully, it's easy to create big piles of meatballs for polypiling by clearing a treasure zoo or graveyard using a footrice corpse , then zapping a wand of striking at the pile of statues.
The obvious use of meatballs is to increase tameness of your carnivorous pets. They will gain a point of tameness (up to 20) each time they eat in your presence. Tame pets are easier to handle and follow you more closely. This is especially important if you are polytrap-dancing for a non-eating pet, or if you are splitting tame puddings.
Interestingly, meatball shares its entry with the huge chunk of meat:
Some hae meat and canna eat,
And some would eat that want it;
But we hae meat, and we can eat,
Sae let the Lord be thankit.