Pak files are essentially archive files that store most of the resources that the game in question requires to run (think of them as custom .zip files)
The game's main pak will always be contained in the Content/Paks directory
Pak files will sometimes be encrypted via AES keys, however these can be easilyfound with AESKeyFinder
Pak files can also be split into chunks for a bit of extra security
Pak files are normally able to be viewed with umodel or Fmodel
Pak files can be extracted using tools such as the ones found in Pakers/Unpakers