Strength
Strength is your melee combats max hit number. As you gain Strength levels, you can deal more damage against opponents. Strength is often favored to train over Attack and Defence by a lot of players, because it allows you to deal more damage.
How to train Strength
In order to gain experience in the Strength skill, you need to have you attack options on the Aggressive option or the controlled option. Aggressive options allow you to hit a little higher over accurate and defensive options. If you choose controlled, you will split your experience through the 3 melee combat skills. If you want to focus on Strength training, you should choose either the second or third option. Be wary to not choose controlled options, as you will cut your experience in thirds.
Strength Bonus
Differently than Attack and Defence, Strength benefits greatly from a stat on gear called "Strength Bonus". When you go to your Equipment Stats page, towards the bottom you will see your Melee strength
Your Melee Strength helps your max hit for your melee weapons. A +1 Melee strength DOES NOT equal +1 strength level. If you want to see the math behind calculating your Maximum melee hit