Magnification
How much closer a scope makes the target appear, expressed as a multiplier. A 5-25x scope ranges from 5x to 25x magnification.
More magnification helps you see the target and refine your reticle hold, but costs you in field of view and parallax sensitivity. For most precision work past 600 yards, 15-25x is plenty.
A fixed-power scope holds one magnification. A variable scope’s range is written low-high, so “3-15x” goes from 3x to 15x. The lower end is for spotting impacts and recovering after shots; the upper end is for the precise hold.