A highsider is something that can happen when you are riding a bike or a motorcycle. It's when you are turning or going around a corner, and suddenly the back wheel of your bike lifts up in the air really fast. This can be really dangerous because you might lose control and fall off.
Imagine that you are at the park riding your bike. You want to turn right to go around a big tree. As you turn, you start to lean to the right while you pedal. Suddenly, your bike hits a bump in the ground or maybe a rock, and your front tire slips a little. This makes your back wheel lift up in the air! You feel like your bike is going to tip over or fall, and you might get scared. This is what a highsider feels like.
In order to avoid a highsider, it's important to be careful when you are riding your bike or motorcycle. Make sure you are paying attention to the road or trail, and try to avoid bumps or objects that could cause a problem. If you feel like your back wheel is starting to lift up, try to stay calm and keep your balance. It's also a good idea to wear protective gear, like a helmet and pads, to keep you safe in case of a fall.