Use WinRAR (official developer) or 7-Zip (open-source).
If the archive contains a mod, it is often best to import it directly through a tool like the Mass Effect Modder (MEM) or the ME3Tweaks Mod Manager rather than extracting it manually. Mass-Effect.rar
Right-click the file and select "Extract Here" or "Extract to folder". Use WinRAR (official developer) or 7-Zip (open-source)