A life point is a measure of vitality on Lyoko. The usual amount for a person is 100 life points, while X.A.N.A.'s monsters and other objects can have more or less. They could be considered the virtual equivalent of life energy, as demonstrated in Laughing Fit.
Under normal circumstances, if a Lyoko Warrior loses all their life points, they will be devirtualized. However, if Aelita was devirtualized, she would have been deleted. After her connection with the Supercomputer was broken at the end of Season 2, she could devirtualize normally.
Life Points can be decreased if a landing was too rough or high, resulting in a reduction of 20. Life points can also be decreased by being attacked by a Monster, with the amount depending on the monster that is attacking and the strength of the attack. Whenever physical damage is done to a certain part of a digital envelope the area crackles with static signifying life point loss.
If a Lyoko Warrior were to enter a tower, their life point gauge would gradually refill if it wasn't already at full.