Your GPA is the average of all the grades you've gotten in school. If you have a GPA of 4.0, it means you've earned all As in your classes.
GPA is an abbreviation for "grade point average," which is typically calculated by adding all the numbered grades you've received and dividing them by the number of credits you've taken. In high school, it's simply a matter of tallying your grades and dividing that total by the number of classes. GPA is usually given on a scale from 0 (an F) to 4.0 (an A), though sometimes it goes higher than that, accounting for a grade of A+.