What is differential braking?
Aircraft with toe brakes have the brake pedals at the top of the rudder pedals.  You press the left one with top of your foot and it applies the left brake(s).  Press the right one to apply the right brake(s).  Normally you press them at the same time and with the same force to keep the airplane from veering to one side (imagine just locking up the brakes on one side of the airplane).



Differential braking is when you press one brake pedal harder than the other.  It is used as an additional form of directional control when:


You have no other forms of steering (either because the steering has failed or there is no nose-wheel steering)
The rudder/nose-wheel steering is not effective enough (because you are going too slow or the wind is too strong).


You want to be careful when doing this, though because you can over do it and create more directional control problems than you originally had.  If you use differential braking during the takeoff roll, it will increase the amount of runway that you use since you are applying the brakes (even though it is only on one side, it will keep you from accelerating as fast).