The attorney only receives a fee if they win your case.  So they are paid only after they win your case. 

This fee is paid out of your potential back benefits.  An attorney can only receive 25% of your back benefits up to a maximum amount after they have won your case.

Your attorney can charge for out of pocket expenses, which are separate and apart from the attorney's fee.  Out of pocket expenses would include expenses such as paying for your medical records from your health care providers. 

You should speak to your attorney beforehand to determine if you will be responsible for paying for these out of pocket expenses.