What makes a safety?

I’m a junior in high school and I was wondering about safety schools. How much lower should their average scores be than yours to make them safeties? What other factors contribute to making them safety schools? Thanks.

U have to be able to afford it, too.

Yes, of course. I’m looking at some state schools and the college my mother works at.

At a minimum, your SATs or ACT scores should be above the 75% for that school. The overall acceptance rate should also be above 40%.

Thank you. That helps a lot.