Backlash is the space between the pinion and ring gear mesh. I don't know the exact number you want but you want the pinion gear to mesh properly with the ring gear so theres not extra stress on the edges of the gear teeth.

Does that make sense? Sorry if i'm not right I can't quite remember.

if you dont know how to do backlash, you can prolly find a guide online, or jsut have a shop do it (ask if they set up rear ends before giving it to them..)
the only tools you need are a tension gauge and a dial indicator. the FSM says how to set everything.