Back pay: You have a date of disability that is when your benefits start. It can be the last date you worked, the day you applied, whatever. Social security office (local to you) can help figure out what to use. That date though is before you apply so when you are approved you get paid from the date of disability up to the day you are approved (minus a few months waiting period) and that is back-pay.

An attorney does not get paid until you win benefits. At that point they get some percentage of your backpay up to a cap. I don't remember what percent but it's pretty low. If you don't get backpay they don't get paid. Some will not take people they don't think will be approved because of that and others will take anyone.