A safety school is one which a college will look at your application and transcript and say, Come on Down, love to have ya! Seriously though, if your GPA is a 3.4, a safety school is one which finds most of its students coming to them with a GPA from high school anywhere from 2.0 to 3.0. Test scores are sometimes asked for, but that would be along the lines of - your overall score on the SAT was a 1100 and the average SAT score of the students accepted to this same school fell around 1000.
There is nothing wrong with a safety school. There is a school for everyone. It is helpful for students to apply to two safety schools while going through the college application process so they can be sure of having a college to attend the following September and making it a bit less stressful while continuing to apply to other schools.
Safety schools are those in which your GPA and test scores are well above those of admitted students, and the school has a high acceptance rate. The college or university may be looking for students with an overall GPA of 3.8 and your GPA is 4.2. A quality school for sure but a bit below your GPA and probably, a tad lower than your SAT scores so, a safety it is. As an example, students who have the grades and scores for a very competitive top-tier school such as MIT, might find that Rensselaer Polytechnic Institute (RPI) or Worcester Polytechnic Institute might be safety schools.
Hope this helps - Good Luck!