In general, a hold (or a "deferral") means that a school is unable to make a decision about you at the time. This means that if better applicants come along, you will be waitlisted or rejected and that if no better applicants come along, you will probably be accepted.
At many schools, hold essentially equals a waitlist spot, with the exception that waitlist spots are at least partially ranked. If you receive a waitlist notification, the schools already has an idea of whether you are likely to ultimately be accepted or not (although they may not tell you this). If you are on hold, everything is still up in the air.