Ground sample distance, usually shortened to GSD, is the distance on the ground between the centres of two adjacent pixels. A 30 metre GSD means each pixel represents a 30 by 30 metre square of the Earth's surface. It is the number most often quoted when people describe how sharp a satellite image is.
GSD is not resolution
The two are related but they are not interchangeable. GSD describes sampling. Resolution describes the smallest feature the system can actually distinguish, and it depends on the optics, the detector, platform motion during the exposure, and the processing chain. A system can be sampled finely and still be blurry.
The practical measure of this is the modulation transfer function, which describes how much contrast survives at each spatial frequency. Two sensors quoting the same GSD can differ noticeably in how crisp their imagery looks because their transfer functions differ. Resampling an image to a smaller pixel size changes the GSD and improves nothing.
There is also a difference between nadir and off nadir. GSD is normally quoted for the nadir view. Pixels toward the edge of a wide swath, or in an off nadir acquisition, cover more ground than the quoted figure.
The tradeoff with spectral richness
An instrument collects a finite number of photons. Every design choice divides that budget.
Making pixels smaller means each pixel gathers light from a smaller patch of ground, so fewer photons arrive. Splitting light into hundreds of narrow spectral channels divides the same photons again. Push both at once and the signal to noise ratio collapses, which destroys exactly the subtle absorption features hyperspectral data exists to measure.
This is why the sensors are where they are. Multispectral instruments with a dozen broad bands can afford ten or twenty metre pixels. Spaceborne imaging spectrometers with hundreds of narrow channels generally sit around thirty to sixty metres, sampling roughly sixty metre pixels across nearly 300 channels. Airborne spectrometers get finer pixels only because they fly far closer to the ground.
Choosing
Ask what size the thing you are looking for is. Mapping alteration across a district works at sixty metres. Finding a specific outcrop or a single leaking valve does not. If your target is smaller than a pixel it does not disappear, but its signal gets mixed with everything else in that pixel, and you move from identification into subpixel detection, which is a harder problem with weaker guarantees.