Referring to two celestial bodies that share (or are close to sharing) the same orbit. The bodies do not orbit each other, per se, instead they follow the same orbit around another body.
A good example is Earth's second moon, Cruithne, which is co-orbital with the Earth. It follows an orbit around the sun very similar to Earth's, only slightly more elliptical. It, like the Earth, takes about a year to orbit around the sun.
The only other known examples of this phenomenon are Janus and Epimetheus, two of Saturn's moons.