Downloading a file named typically refers to a compressed archive containing data related to the T.38 Fax-over-IP protocol or, more commonly in hobbyist circles, digital assets for the Northrop T-38 Talon aircraft used in flight simulators.

You cannot open a .rar file with standard Windows or Mac tools alone. You will need a third-party "unarchiver":

Most "t38.rar" files found on community forums like the Microsoft Flight Simulator Forums contain 3D models, cockpits, and flight dynamics for the Northrop T-38 Talon aircraft.