Hacking is considered a mini-game in Fallout 3, which if completed will open safes, turns off turrets,or even open doors. Make sure you put points towards your characters science skill to aid in inputting the right code.
Simply activated a locked computer terminal will activate the 'hacking' feature. You must guess initially, then follow the clues on the bottom to guess more effectively. The higher your science score, the less options appear, making your chance of success higher.