Alt-strumming (or Alternate Strumming) is a frequently-used technique that is designed to make fast sections of songs much easier to play. When one alt-strums, they move the strum bar in both directions (down and up), as opposed to just down. While the technique can be used at any time, it should be used in fast sections of a song or when strum notes are too close to each other that makes only-down strums impossible.
Examples of songs that require the player to utilize alt-strumming a lot include "Knights of Cydonia", "Through the Fire and Flames" (and any other DragonForce song), "Pull Me Under", "Assassin", and "Fight Fire with Fire".
Then there are songs like "Laser Cannon Deth Sentence" that require the user to practice alt-strumming on different enclosed strum notes and to practice alt-strumming triplets. Sometimes, during a stream of even-numbered strum notes (2, 4, 6, 8 notes, etc.), the player would have to end a strum on an up-strum if they started on a down-strum or vice-versa, and strumming triplets would flip the player's strumming rhythm on beat (from down-up-down to up-down-up for the next beat).