Concurrent enrollment is a process through which students may take courses for college credit at a local community college. The student's high school (as this is usually only done for grades 11 and 12) pays for some amount of credit hours and it is worked into the student's schedule that he or she will not be at the high school at those times.
Many colleges (mostly state colleges) accept the credits earned in this process, so that the student gets a head start on their core requirements when they attend college. Of course, these courses also count for high school credit.