For instance, a single standard vertical aerial will send out signals 360 degrees all around but not really up or down. A signal will hit the Atmoshpere from the radiation angle, which is simply the angle at which the waves are travelling. From the top tip and bottom tip of an aerial is the least signal coming out, like the light from a flourescent bulb. When this same aerial is sitting horizontally, all though the pattern doesn't change that much the waves are now up and down, instead of all around. The tips of this aerial will again transmit the least.
Polarisation is whether radio waves are transmitted horizontally or vertically.
A possible explanation for this is to say that the radio waves are on a different angle to the aerial. If you have a horizontal aerial, and a signal is coming from a vertical aerial the radio waves will clash with the aerial, and the aerial will not absorb the the waves at their full potential and thus conduct the signal down the co-axial cable to the radio. It's the same for horizontal signals being transmitted to vertical aerials. The diagram on the left represents an aerial standing vertically emitting radio waves. The diagram on the right represents an aerial sitting horizontally emitting radio waves.