junior is first year subjects.
senior is not first year subjects. These often require a min of 12 credits of junior subjects as a pre-requisite. In some schools within the arts faculty you have just junior and senior, in others, you have junior, senior and advanced. eg for English there are junior, you need to have completed 12 credits of junior subjects to go onto senior, and then you need 36 senior credits to get your major. However, if you have a credit average in at least 12 credits of senior subjects, you can do Advanced subjects which are generally harder, but far more rewarding. You can complete your major of just junior and senior, or of junior, senior and advanced units of study.
Things are a bit different in the science faculty where you usually have to complete junior subjects as pre-reqs to intermediate ones, and then the intermediate ones are pre-reqs for senior ones.