You will need a utility like WinRAR or 7-Zip to open and extract the contents.
If you are installing the full game, you can expect the following core content as detailed by IGN :
Large files containing the actual game assets, such as the city of Rome, character models (Ezio Auditore), and audio files.
A file named Assassins-Creed-Brotherhood.rar is a compressed archive that typically contains the installation files or digital assets for the 2010 action-adventure game .
Essential software like DirectX or Visual C++ required to run the game.
Files with the .rar extension from unofficial sources can sometimes carry security risks.