There are three types of decisions that you can receive at a university. They will either accept or reject you; or put you on the waitlist. Waitlist is usually a sign that they are interested in having you on their campus, but they could not decide based on the information and availability they have. There can be multiple undisclosed reasons you're waitlisted; all you can do at this point is wait.
Some universities allow you to submit an additional essay expressing any change in academics, experiences, or family situations between the time you submitted the application and when they put you on the waitlist. It is imperative to submit this letter. They'll review your application once again while selecting candidates from the waitlist, so having this letter will provide an advantage. Many students do not submit this essay because they are either tired of the application process or have already accepted an offer somewhere else.
It is also a good idea to contact the university's admission office and ask them if you can do a video call with them. Doing this shows your interest in the university and gives the admissions officer a chance to know you personally. This way, they can put in a good