Countersteering is a way to turn a bicycle or motorcycle. You know how you turn a bike by turning the handlebars to the side you want to go? Well, that's not always the best way to turn when you're going fast, like when on a motorcycle.
When you're moving fast, it's hard to turn the handlebars enough to make a sharp turn. That's where countersteering comes in. Instead of turning the handlebars the direction you want to go, you actually turn them in the opposite direction for just a moment. This causes the bike to lean in that opposite direction and then, like magic, turn the direction you want to go.
So, if you want to turn right, you actually turn the handlebars to the left for a split second before straightening them out again. This causes the bike to tilt to the left, and then the wheels naturally follow, turning you to the right! It may sound a bit strange, but it's really how it works.
Countersteering helps you make turns quickly and safely when you're moving quickly. Remember, though, that it takes practice to get it right.