Wikipedia
Repurchase agreement
A sale (and) repurchase agreement, also known as a (currency) repo, RP, or sale and repurchase agreement, is a transaction concluded on a deal date t between two parties A and B:
(i) A will on the near date t sell a specified security S at an agreed price P to B
(ii) A will on the far date t (after t) re-purchase S from B at a price P which is already pre-agreed on the deal date.
If we assume positive interest rates, the repurchase price P can be expected to be greater than the original sale price P.
The (time-adjusted) difference (P-P)/P/(t-t)*365 is called the repo rate; it can be interpreted as the interest rate for the period between near date and far date.