GPA stands for Grade Point Average. It is a standard way of measuring academic achievement in the U.S. Basically, it goes as follows: Each course is given a certain number of "units" or "credits", depending on the content of the course. In secondary school, most courses carry the same number of units, but this is not true at the college level. Most college courses have a load of three units (approximately three hours of lecture and six hours of homework per week for each semester), but the number can vary from fractions of 1 to more than 5. GPA assumes a grading scale of A, B, C, D, F. Each grade is assigned a number of grade points. An A grade receives 4 points, a B=3, a C=2, a D=1, and an F=0. The process is the same whether you are working on secondary or college grades.
If you take a three unit class and receive an A grade, you receive 3 units times 4 points (for the A), which gives a total of 12 grade points for the course. Let's say you also take a 4 unit class (common in Mathematics, for example) and receive a C grade. That's 4 units times 2 points for 8 points. So for your two classes you have accumulated 20 grade points for the 7 units. You then divide the accumulated grade points by the number of units and you have your GPA. (20/7=2.86), so your GPA is 2.86, which is slightly less than a B average.
If your grading system is A-F, with a 4 point scale, you may be able to calculate your grade point average. However, most systems outside the U.S. do not use that scale. Even some systems that do use a similar scale have different meanings for grades (for example, in the U.S., "A" grades are not rare, but in some systems they are almost never given), so the GPA is not representative of the same thing that it is in this country. It is for that reason that colleges here usually have people specially trained in the evaluation of credentials from other countries