Deferring your admission means you have been accepted but have decided to change the start date to a later semester. It's pretty common, for example, for a student to get admission for the August semester and defer it to the following January semester. You can do this either for medical reasons or if your university allows it for personal reasons.
In some cases, students decide to take one full gap year and defer their admission to the following year. Most recommend only taking this one year off if you have something productive that you want to do (e.g., volunteering across the world, doing an internship, starting a business), etc. These are great examples of activities to do during a gap year. Most universities might ask for proof of what you'll be doing during that time off, so be mindful of that. Lastly, if you have any type of scholarship from the university, you'll need to make sure you can defer that as well.