If I was read my Miranda rights but was not arrested, what does this mean?

Since I was not present and do not know any details of your situation I can only reply theoretically. It may have been that the police were considering taking you into custody and were protecting themselves and your constitutional rights by providing you Miranda warnings that you had the right to remain silent and have an attorney present for any questioning. Once you are detained, even if not arrested, the police must provide these warnings for any testimony that you may provide to be admissible. Since they did not arrest you it essentially becomes moot.