A safety school is a college where you’re convinced you’ll be accepted to. If for some horrible reason all of your other schools turn you down, you can count on your safety school to accept you, so at least you’ll have some place to go. Depending on how good your high school resume is, how selective the schools you picked are, and how many colleges you’ve applied to, you’ll almost definitely get into a school that’s better than your safety school. But it’s good to have a fallback option.
Unfortunately, a lot of students don’t put much thought into choosing a safety school and just pick the nearest college that they are all but guaranteed in to. This is actually a bad idea, because if you do end up going there, there’s a good chance you won’t like it.