A college credit is a unit of work you receive for completing a course.
At some schools you can take a test to receive credit prior to starting school, but the majority of credits are received by passing courses or transferring them in from another school.
The amount of credits required to graduate depends on the major you select.
At most 4-year schools, a course typically counts for 3-4 credits, with some specialized offerings counting for a bit more or less.