Resolution is the ability to distinguish between two things that are close together.
Spectral or energy resolution is how well a telescope can differentiate between two light signals that are very close in energy or frequency. The higher the resolution, the closer two signals can be while still being recognized as distinct. Resolution is an important instrument characteristic in astronomy. An imaging telescope with very low spatial resolution will not reveal small features such as a star's nearby companion or the fine structure of a supernova remnant. Instead, they will get blurred together.