Pounds per square in gauge, or psig, is a measure of pressure, specifically it is a gauge pressure. This means that it is measured with respect to the atmospheric pressure. Psig is used in contrast to psia, which is pounds per square inch absolute, measured with respect to vacuum. This can all be visualized in Figure 1.
For example, a bike tire may be 65 psi above atmospheric pressure which is its psig value, and if the atmospheric pressure is 14.7 psi, then its psia value would be 65 + 14.7 = 79.7 psia.