Interest is a charge applied to borrowed money, and is generally expressed as a percentage per year.

When buying CFDs, you are in principle borrowing money for a trade, so you are charged a standard interest rate based on the current LIBOR (London Interbank Offer Rate) plus a small percentage.

When you sell a CFD, on the other hand, you receive interest for the amount the CFD represents. This percentage is the LIBID rate minus a small percentage.