Spectral resolution is a very important parameter in optics. Here you can learn more about it.
The spectral resolution describes the device's precision to distinguish the narrow ranges of wavelengths. For example, if our spectrophotometer with wavelength range of 200 nm had a spectral resolution of 1 nm, the system would be capable of resolving a maximum of 200 individual wavelengths (peaks) across a spectrum.
Two wavelengths usually are considered resolved if the minimum between the two peaks of the detector output signal is lower than 80 % of the maximum. This condition is known as the Rayleigh criterion.
Spectral resolution is a parameter that is unique for each spectrometry system. It depends on many components, such as the monochromator- the width of the slits and the diffraction grating, the lenses and the detector.
For measurements in the spectrophotometer, the optimal resolution is determined by the spectral shape of the sample. The light intensity reaching the detector can be enhanced by slightly opening the slit, however it affects in poorer resolution.
The important parameter when defining a spectral resolution is FWHM. If two peaks are overlapping FWHMs, it means that they are indistinguishable.
References
1. https://en.wikipedia.org/wiki/Spectral_resolution