A repo rate is the gain in price between the two trades in a repo transaction (i.e. the difference between the sale and repurchase prices) as an annualised percentage of the sale price.
If you regard the transaction as a secured loan and repayment secured against a security (which is the economic effect it has), the repo rate is the interest rate on the loan.
Central bank repo rates are an important indicator as repo transactions are a key (usually the major part) of their open market operations.
Central banks usually publish current and historical repo rates, while market rates for sterling and euro transactions respectively are available from BBA Libor and Euribor.