question about back spacing

[mcgarrey]

hello yall, i am looking to put a set of 315 75 16 on my 97 toyota 4runner, i have a 3 inch suspension lift and a 3 inch body lift i will also be putting on, i was speaking to some one on here and they mentioned back spacing. i know nothing of back spacing so can some one give me a run down and perhaps what i may need. Thanks