In practice it is how small are the details you can see with the lens. Resolving power is an instrument property specifying the smallest detail that a microscope can resolve in imaging an ideal specimen. Resolution refers to the detail actually revealed in the image of a real specimen.
High resolution refers to small values of the minimum resolvable distance.
Of course, this limit requires ideal lenses under ideal conditions. As one goes to higher and higher magnification, the depth of field in the sample gets smaller and smaller. It becomes hard to keep the entire specimen in focus. The magnifying power M-lens of a simple lens viewed by eye is given by the ratio of the apparent size of an object, seen at a distance of 25 cm from the eye to its actual size. When using a simple microscope it is useful to have a comparison of distance in view at the same time.