The definition and function of a convocation or a convocation ceremony on a college campus depends significantly on the institution itself.
At some institutions, "convocation" refers to a ceremony at the start of the academic year, usually in the fall, where new students are formally inducted into the community. There is often much pomp and circumstance, with the entire college community involved. Students may be given a small gift, be asked to sign a book with the names of all new students, or even have their parents participate in some kind of symbolic ritual.
At other institutions, "convocation" may refer to graduation ceremonies -- in two ways. First, "convocation" might simply be another term used for "commencement," which is the official graduation ceremony. Second, "convocation" might also be used for a supplemental graduation ceremony; "commencement" or "graduation" might be used to describe an institution-wide graduation ceremony, for example, whereas "convocation" might refer only to the graduation ceremony for your particular college (or even academic department).
Because of the multiple meanings, "convocation" can indeed be confusing. If you're not sure what it refers to at your particular school, don't be afraid to look online or even ask someone. You are definitely not the only person who is unclear on the concept!