GPA is short for Grade Point Average. An A equals 4 points, B 3, C 2, D 1 and F 0.
If one were to take 4 classes, all of equal weight, and receive a B, 2 Cs and an A, that would equal 11 (3 2 2 4). Divide 11 by 4 to get the GPA; 2.75.
In college, it's a little different because the classes have different credit values. Most classes are 3 units (credits) but some are 4, 2 or 1...rarely 5 or higher. So, if we use that same example (3 2 2 4) with all 3 unit classes, it's still 2.75. But it's different when the classes have a different value, of course. For each unit, another grade point equivalent is added. Here's an example.
Eng 15 - 3 units - A - Grade points 12
Span 1 - 4 units - C 8
Bio 20 - 3 units - C 6
Math 4 - 3 units - B 9
You add all the grade points (35) and divide by the units attempted...in this case, the GPA is 2.92.
I hope that wasn't too much useless information. LOL!!