In economic terms, the opportunity cost of doing a particular thing is what you give up in order to do that thing. We all have scare resources so any time we do one thing, we lose the opportunity to do another thing. With this in mind, let us think about what this economist means.
One aspect of the opportunity cost of going to college is the price that the student pays to attend the college. The student could have used that money for something else. The ability to buy those other things is part of the opportunity cost of going to college. However, that is not all that the student gives up. If the student did not go to college, he or she could presumably have gotten a job. In order to go to college, the student is giving up the money they could have made by working. They might be giving up the opportunity to spend more time on hobbies than on studying. They might be giving up the opportunity to get married. All of these lost opportunities are part of the opportunity cost of attending college. Therefore, the opportunity cost of attending college is higher than the monetary cost of doing so.