Battlefield 3 gives the player an option to install an HD Texture Pack to their hard drive for a gigantic 1.5 gigabytes of hard drive space. The comparison above shows the difference between the regular textures and HD textures.
From the results of the video, the HD textures are almost mandatory for a High Definition screen, as declining the install will make the game's textures look fit for a PlayStation 2 title. When the pack is installed, it will stream these quality textures from your hard drive and quickly fade in; a process almost invisible to players while shooting hoards of bad guys.
The pack is most likely due to disc space limitations. The single player disc, when installed to the hard drive, is a gargantuan 7 gigabytes, which, even with the new disc format, is not enough to store the high-rez textures.