Will is a resource that is used up in the Gym. Your spartans will can be improved via special 'chips' or moving to a better vehicle.
Will is drained each time you train so it is imprtant to keep it at a high level (the higher your will level, the more stats you gain from training). You can refil your will bar in the gem temple (explore>base exchange>gem temple), or you can use Adrenaline Shots .
Your Will level is someitmes drained in scouting turns.