That really depends on what is inside the tar file. If there is one or more apk archived in there, you just need to extract it from the archive. If what's in there isn't a apk then the question wouldn't make sense.
A tar is a "tape archive" file. I usually unpack these using a command line, e.g. "tar -xf tar.tar". That will work for sure on linux, should on OSX, but to be honest I don't really use Windows. If not you'll have to download and install an app to unpack them.