According to the article, the collection preserves every game turned in for copyright registration, which works out to be about 10% of the games published each year. They collect the games themselves, along with promotional materials and guides associated with each game.
According to the Moving Image Materials collection policy listed on the LOC's website:
"Video games have become an established, popular medium of moving image entertainment which demand inclusion in the collections of MBRS. The Division is developing new approaches for the more systematic acquisition of video games, including playback consoles and platforms, the multiplicity of formats and their equipment needs, and the technical challenges in preserving the digital source files. The collection will encompass a wide range of examples of video game culture, to allow historians decades hence to fully understand this as a popular phenomenon, and not have simply a few games which seemed significant at the moment of release."
Read the article here.