Imagine you are fishing on a lake. The sun is beating down and glaring off the water. You take out your polarizing sunglasses and rotate them while looking at the glare, and at one rotation the glare goes away, and another rotation 90 degrees from the first, the glare is strongest.
What you are seeing is a polarized electromagnetic wave (sunlight) and you are filtering out or letting in that one polarization.
You can rotate some antennas, like a dipole, the same way, and capture a lot of the transmitted RF signal (when the antennas are in-polarization), or capture very little of the transmitted RF signal (when the antennas are cross-polarized).