Since everyone is guessing - here’s my guess. A safety is probably a college in your region that is very familiar with your school and often has some or many kids enrolled from your high school that are similar in caliber (e.g. top 25% or whatever criteria fits the student) and that you can afford.
Anecdotally for my #2, NAU (Northern Arizona) was his “safety school”. That is along way from Michigan, no one from his high school in the history I don’t think has gone there and NAU took a looooong time to admit him. Granted he didn’t attend but it was one of of those safeties he could have attended because it had everything he wanted including some depth to this particular major and interest… We visited, talked to admissions, he talked to a prof…don’t know what more a kid can do. NAU has auto admit at his caliber and they still didn’t admit him until spring despite an October application…so much for getting your safety under the belt early. I think it was his higher than the autoadmit GPA and out of the area that stalled them off admitting even with rolling.
Second Anecdote, the year he graduated from the school her chose (in 4 years) he started getting info from NAU on their graduate programs so they definitely use a CRM system that tracks applicants.