Resolving power, measured in arc-seconds, is the ability to discriminate between two close points, such as double stars, or to reveal detail. The celestial sphere is divided into 360??, each degree is divided into 60 "arc-minutes." These arc-minutes are further divided into 60 parts, each called an "arc-second." From directly overhead (0??) to the unobstructed, or "true" horizon (if on a plain) is 90??, or one-quarter of the celestial sphere. The following is an example of how the theoretical limit of resolution can be applied.
If the telescope can resolve 1.0 arc-seconds, objects 1.0 arc-second apart can be recognized as two entities. The Moon measures about 0.5 degree, or 1,800 arc seconds. If enough giant beads were lined up at the visible side of the full Moon's equator, each just 1.0 second-of-arc from the next, they would be distinguishable as circular bulges resembling a string of beads belting the equator. But, if the number of these giant beads were increased so they were separated by only 0.8 second-of-arc, the "string of beads" would change to a band with fuzzy edges. Telescope manufacturers always list the theoretical capability of resolution. Some meet or exceed that design limit, some do not.