Here is the explanation.

Typically, Likely Letters are sent to applicants several weeks before official admission verdicts are slated to go out. This usually means some time in October for Early Decision/Action applicants and late-February or March for Regular Decision students. The gist of these missives is: “We fully plan to accept you so you can breathe a sigh of relief, but don’t screw up between now and when you get your official acceptance because this one isn’t quite official.”

Most commonly, the term “Likely Letter” is associated with the Ivy League and with athletes … and with good reason. These letters are one tool that the Ivy League schools use to encourage their recruited athletes to put other options on the back burner. Although coaches tell admission officials which prospective players should get these letters, it is the admission folks themselves who make the final determination and send the letters out.