Alt Rock is a broad category of popular rock music typically regarded as somewhat out of the mainstream and variously including elements of punk rock, heavy metal, folk music, etc. Think of bands like Airborne Toxic Event, Guster, The Oh Hellos, The Fray, The Lumineers, and The Decembrists.
Good solid rock music that is a bit eclectic. You don’t find them on stations that play Beyonce or Adele, or Taylor Swift, but definitely on college rock stations.
In the old days we used to call it “album rock” because DJ would play a whole side of an album.