Every day bungie releases a set of 4 daily challenges for the community to try and unlock.
They can be found here: (And reset every day at 6AM EST)http://www.bungie.net/stats/reach/globalchallenges.aspx
You can also see these in game, by pressing the start button and scrolling down to challenges. The 5th and last one on the list is the weekly challenge, and it is not required.
Your goal is to complete all 4 goals in one day. Once you complete them all the achievement will unlock.
Most likely you will be asked to play all 3 modes (Matchmaking, Campaign and Firefight.) For achievements that are not gametype specific, it is usually easiest to play Firefight by yourself, on at least normal difficulty.
NOTE: When viewing the challenges in-game, you can press "A" on them, and the game will take you to the correct playlist.