PSI and PSIG are both units of measurement for describing the amount pressure a gas or fluid is exerting. However, PSIG specifies what the measurement is relative to, whereas PSI does not. In both units, the letters "psi" are an abbreviation for "pounds per square inch."
PSIG stands for "pounds per square inch gauge," or gage. PSIG units are relative to atmospheric pressure. The air surrounding Earth exerts pressure on all objects on the surface of the planet. At sea level, this pressure is 14.7 pounds for every square inch column of air. PSIG units use this amount of pressure as a baseline, whereas PSIA units (pounds per square inch absolute) use a vacuum as a baseline because vacuums have a total lack of any atmospheric pressure.
Most measurements of pressure made on the surface of the Earth are actually in PSIG if they do not specify what the pressure being measured is using as a baseline. For example, a basketball pumped up to 8 PSI is actually pumped up 8 PSI units over the ambient atmospheric pressure and would have 22.7 PSI if measured relative to a vacuum.