Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Jax is the strong fighter with a good ability, equipment mastery, which makes him has more health due to his increase damage and ability power.
The first ability is Leap Strike
He jumps to the target and then deal bonus damage if it's enemy. You can use this move not only for attack, but can also to flee from the enemy, such as jump on to your minions.
Next ability is Empower
He charges up his power and if he attacks while he is charging, he deal bonus magic damage and can damage nearby enemy. You can use this ability before you use Leap Strike, so you can combo the bonus damage of Leap Strike and Empower together in 1 attack.
The third ability is counterstrike
He increases his dodge chance, he can active this ability while he is dodging to deal damage to surrounding enemy and stuns them.
The last ability is Relentless Assault
Jax grows stronger and increases attack speed every successful hit, and every third attack he deal bonus damage. This ability is good for solo fighting with the enemy because if he continues attacking the enemy(if the enemy is not leaving), he gets advantage by growing stronger and every 3rd attack he deal more damage.