Health or Hitpoints (HP) determines how much damage the player or a monster can take before being defeated in combat. The Player's maximum HP can be increased by 1 for each Combat Level after unlocking the Combat Research Level 1. In addition, there are two Heart Crystals that can be obtained from Lava Aliens and Skeleton Monks, which will each increase the maximum HP by 5. The current maximum possible HP, with Level 100 Combat and both Heart Crystals, is 120.
Speed determines the rate at which attacks are performed during combat. Combat proceeds with a series of ticks of approximately 1 second. The speed stat directly corresponds to the number of ticks between attacks, so a weapon of speed 3 will attack every 3 game ticks. A lower speed will attack faster and a higher speed will attack slower. If your weapon has a different speed than your opponent, your attacks will become offset, which can be used as an advanced combat technique.
Attack determines the Maximum Hit of a weapon. During the combat calculations, if you land a successful hit, the game will next calculate what the damage value is, which is randomly chosen from 0 to the Maximum Hit. Notably, the Offhand Dagger and Offhand Dagger+ are weapons that gives an Attack bonus that is not equipped in the on-hand, and so increases the Maximum Hit of any weapon by +1 And +3 respectively
Accuracy increases the Hit Chance, or the probability that an attack will hit successfully. The Hit Chance also depends on the opponent's Defence. If your Accuracy is more than double your opponent's defence you are guaranteed to land a successful hit (however, because a successful hit can damage from 0 to Maximum Hit, you can still hit 0's).
Defence decreases the Hit Chance of your opponent on you, making it less likely for them to land a successful attack. By decreasing your opponent's Hit Chance against you, you can significantly decrease the amount of damage you take during fights.
Note: Because magic spell damage is calculated differently, the Defence stat does not seem to affect damage received from magic spells.
Magic Bonus provides special bonuses for many Spells when cast, often increasing the damage or duration of spells with certain Magic Bonuses. Unlike the other five Combat Stats, Monsters do not have a Magic Bonus, so it is unique to the player.
Weakness is a special statistics that only certain Monsters have, that can have different effects depending on the Monster, but generally means that you will deal double Damage against them if using an Attack Style or Weapon that is against their Weakness.