If you're willing to sit through the long explanation...
Many employers will set up a tiered system where the first block of work done in any given month is paid at a certain rate and then the next block is paid at a higher rate (presumably because the first block partially went to pay for overhead (assistants salary, rent, etc)
So, the incentive is to reach the higher tier because you get paid more for each patient, kind of like overtime.
You will learn what an wVU is at some point (work relative value unit. It is what each patient encounter reimbursement is based upon.
e.g.
simple visit, established patient 99213 worth 0.67 wRVU
more complicated 99214 worth 1.1 wRVU
OB care and delivery worth 23 wRVU
so the employer may pay $25 per wRVU for the first bit permonth, if you see more patients that month once you hit a certain threshold each wRVU may be worth $35, then you'll hit another threshold if you see even more patients and the last tier may be worth $50 per wRVU because by then you've paid your overhead.
If an employer is smart they'll keep your base salary low to incentivize you to see more patients to get production pay.