From Mechwarrior Living Legends Wiki
Jump Jets, often abbreviated to "JJ", enable a Mech or Battle Armor to become airborne briefly. They may only be fired for a limited time (indicated above the speed gauge in Mechs, and next to a Battle Armor's armor/health status)and require time to recharge. They also generate significant heat.
The Final Beta 0.7.0 release introduced a new kind of JJ, called improved Jump Jets, or iJJ
Jump Jets provide Mechs with several new tactical options:
1. Accessing terrain features too steep to climb.
2. Quickly jumping to cover behind low hills.
3. Briefly rising from behind cover to assess the battlefield.
4. Jump Sniping (aka Poptarting) from behind cover to fire weapons.
5. Avoiding - to an extent - incoming missile fire.
Improved Jump Jets trade much of their vertical thrust and a higher weight for better horizontal acceleration, much better control and better control over fuel consumption. Particularly advantageous is the ability to thrust sideways rather than only forwards and backwards.
This greatly increases their ability in combat related tasks, like jumping to cover or dodging missiles but at the same time are less useful for Jump Sniping and terrain that requires significant upward travel.
A video showing the differences between both kinds can be found here: Jump Jets Preview
The heat generated by the use of both kinds of Jump Jets needs to be carefully considered in the overall management of a Mech's heat. For example, firing energy weapons mid-jump could result in a heat spike which damages your Mech.
Jump Jets are a key Battle Armor defense. Jump Jets accelerate Battle Armors faster and further than Mechs. This agility can make a skilled Battle Armour more difficult to hit.