A ring oscillator is a circuit that self oscillate, i.e. it oscillate by itself. It's composed of an odd number of NOT gates chained together in a ring arrangement, the last one being fed back to the first. The frequency of oscillation depends of the propagation delay of each gate. Being composed of an odd number of gates, the output of the chain will always be the logical NOT of the input.
This output, fed back to the input causes the chain to always invert its state.

If you want to know more, please visit wikipedia (where I borrow this drawing) which explain it better than I do.