Cash on Cash Return (CoC) is the cash income returned on the cash invested.  To calculate the cash on cash return, take your net cash flow (after all expenses) and divide it by the cash initially invested.
Example:
$100,000 property purchase
$30,000 cash invested for down payment, closing costs, rehab, etc..
$12,000 Annual Gross Rent
$6,000 Annual Expenses (including debt service)
$6,000 Annual Net Cash Flow
CoC - $6,000 / $30,000 = 20%