What Is It?

Urban Dictionary:
The "Konami code" is a cheat code originally designed to be used with an NES controller in Konami video games from the mid-1980s. Over the years it's found its way into many other game systems and OSes. It represents the buttons that must be pushed in this order: 

Up Up Down Down Left Right Left Right B A 

The code usually gives the player a full powerup or extra lives. It's also often called the "Contra code" because that game was considered so difficult that many believed it was impossible to win without using the cheat code.

Source: http://www.urbandictionary.com/define.php?term=uuddlrlrba

The Konami Code, known in Japan as the Konami Command, is a cheat code that appears in many Konami video games, although the code also appears in some non-Konami games.  The code was first used in the 1986 release of Gradius for the Nintendo Entertainment System but was made famous in North America in the NES version of Contra, for which it was also dubbed the "30 Lives Code". While pausing the game or during the title screen, the player could press the following sequence of buttons on the game controller:

↑ ↑ ↓ ↓ ← → ← → B A

The code was subsequently re-used in a large number of other games, and has found a place in popular culture as a reference to the third generation of video game consoles.

Source: http://en.wikipedia.org/wiki/Konami_Code
List of Konami code games: http://en.wikipedia.org/wiki/List_of_Konami_code_games