The figure below depicts a caliper. It can measure lengths from 0 to 7.5 inches to a precision of one thousandth of an inch. One can measure the outside of a part with the jaws, the inside of a hole or slot with the nibs, or the depth of a hole or shoulder with the extension bar. This particular one is has a vernier scale. It takes a little practice to read it properly.
Calipers often have a dial or digital readouts instead.
Read the small number division. Read the number of smaller subdivisions. Each represents 0.025 inches to be added to the measurement. Read which line on the vernier lines up with a line on the main beam. For each line a thousanth must be added to the measurement.