Ensure the post is from a trusted community member. RAR files can easily contain executable malware disguised as data.
Small "ST3K" or "STK" tools often refer to "Service Tool Kits" or specific automation scripts. ST3K.rar
Because .rar files are containers, the specific contents depend entirely on the context of the post where you found it. Common associations for this type of naming convention include: Ensure the post is from a trusted community member
If you have already opened it in a safe environment (like a Sandbox), look for a .txt file inside to explain the contents. ST3K.rar