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A telescope amplifies light by taking in light through an opening much larger than your eye's iris. Your iris is just a few millimeters across, but a telescope should have a much, much larger opening or "aperture" for best light amplification. A good size telescope for first-time astronomers should have at least a 90 millimeter aperture, in my opinion.

Magnification of a nighttime telescope is also important, but it is secondary in importance to light amplification. Any telescope can magnify as much as you wish (up to a useful limit) simply by changing its eyepiece. The useful limit of magnification is larger if the telescope has a larger aperture.