Read up on Yagi antennas.

Essentially you are stacking waves. If you have an array of trasmitters, you can have them send a constructive signal or a destructive signal as a signal "wave" passes them. Using this property, you can change the shape of the wave propagation. Think of it like throwing a stone in a pond, and then throwing in a second or third stone at the exact right moment to combine the ripples, creating a stronger wave in a particular direction depending on when and where you throw the stones.