Stats

HP
HP, or Hit Points, is your character's vitality and ability to sustain attacks. This starts at 20, and only increases if you are on a Genocide or Neutral route when you kill monsters and level up. It should only rise above 20 on a Pacifist Route after sleeping at an inn.

When your Soul is hit by a monster's attack, HP is reduced, and when HP reaches 0, the heart shatters, and it is Game Over.

HP can be restored through various consumable items or through interacting with a save point, and can be risen past its maximum by sleeping at an inn.

Attack
Attack (or AT) is a stat that measures the rate of which you do damage. This is measured for both you and monsters. AT can be increased by using different weapons, but also by using certain ACT options and eating certain items. AT can be accounted for with a large amount of DF.

Defense
DF is a stat used in Undertale that effects both you and enemies. It stands for Defense. The greater the number of DF a character has, the less damage you take. You can change your defense by either gaining LOVE or by wearing Armor. Certain ACT commands and Consumables will also effect your DF, as well as maybe effecting the enemies DF.

EXP
EXP, though often known to stand for "Experience Points," truthfully means "Execution Points," as explained by Sans in the Final Corridor. Getting enough EXP can increase your LV, or "Level of Violence." This makes it easier for the player to kill monsters and gives them more HP.

In the Pacifist Route the player does not gain any EXP because they do not kill any monsters. However, during a Genocide Run the player gathers a mass of EXP, resulting in a very high LV rank. This is not necessarily a good thing if the reactions of NPCs is anything to go by.

LOVE
LOVE (also known as Level Of Violence) is the leveling system in Undertale. It is also abbreviated as LV.

As the player's LOVE increases, their hit points, attack and defense stats increase.

The player starts the game at LV 1. Earning EXP from killing monsters increases LV. If the player intends not to kill anyone, their EXP will remain at 0 and their LV at 1.

In the demo, EXP can be gained from hurting monsters alone - killing them is not required. However, in the full game, EXP can only be gained through killing.

Main Story
Flowey, in the beginning, tells you that LV stands for LOVE. He tries to use this to trick the player into walking into his bullets 'friendliness pellets'.

On neutral and pacifist runs, Sans reveals what LOVE and EXP stand for in the Last Corridor; he explains that the term LOVE is an acronym for Level Of Violence, and that EXP stands for Execution Points. As each of these increase, then so does the character's proficiency in murder as well as the ability to emotionally separate themselves from the acts that they are committing.

On a pacifist run, Sans states that the player never gained any LOVE, but has instead gained love.