A microscope's resolution measures how much detail a user can see. A microscope may have powerful magnifying lenses, but if the resolution is poor, the magnified image is just a blur. Resolution is the shortest distance between two points that a user can still see as separate images under the microscope.
A compound microscope can't distinguish details closer together than 200 nanometers. The most powerful electron microscopes go down as low as .2 nanometers. A microscope loses resolution if the lenses aren't perfectly aligned. Viewing light with shorter wavelengths produces better resolution than longer wavelengths. There are mathematical formulas that use the wavelength and the numerical aperture -- the microscope's ability to gather light -- to calculate resolution. Specimens in which the different parts aren't very distinct may give the user a poorer resolution, even with the best microscopes.