How does RF anti-jam technology work?
One method is by actively steering the antenna (mechanically or electronically) to place a "null" in the direction of the jammer, reducing its signal strength significantly, while affecting the desired signal minimally, if at all.

Also, assuming the jammer signal strength isn't so strong that it saturates the receiver front end, advanced DSP techniques can be used to estimate and cancel the effects of the jamming signal. The communications protocol itself can be designed to optimize the ability to do this. The problem for the jammer is to mimic the desired signal closely enough to confuse the anti-jam algorithm.