Scrubbing is when an advertiser doesn't pay for ALL of the submitted leads. For example, they tell you you're getting paid $5 CPA, but they don't tell you that they're NOT going to pay you for leads that come from certain states. You still send all of your traffic to an offer, but you simply don't get paid for all of the leads. Scrubbing is typically frowned upon, but is very common in many lead-gen verticals.
For example, if you look at any mortgage offers out there, they typically don't pay for "NEW PURCHASE" leads, but a consumer can still submit them, however, they WILL accept Refinance leads.
You should look for "no scrub" campaigns and if that is not stipulated, you should ask whomever you're working with which leads (if any) do NOT get paid for. Especially if you are doing PPC, you can set up negative keywords for the leads you wouldn't get paid for anyway, or find another advertiser that will accept them.
Make sense?