iirc beat up is a 10 BP move which goes only on base attack vs base defense. Each pokemon on your team that is not fainted or with a status effect will attack the opposing pokemon with a 10 BP (might be higher) attack going with base attack vs base defense.
It ends up quite weak. Pretty much the only pokemon you can use it on is Blissey, since Beat Up only goes by base defense and not EVs or nature. Most of the time it can 2HKO Blissey, which is a good move Houndoom and any Jolly or Choice Scarf Dugtrio.