Unilateral training means working one side of your body at a time, like your left or your right arm or leg. When you do exercises like squats or lunges or push-ups, you are using both sides of your body at the same time, but when you do unilateral exercises, you do things like one-legged squats or one-arm push-ups, and this works just one side of your body.
Why is this important? Well, it helps you to make sure that both sides of your body are equally strong and balanced, which can keep you from getting injured. Sometimes, when we do exercises that work both sides of our body, we might not realize that one side is stronger than the other, and we might end up relying on that stronger side more than the weaker one. Doing unilateral exercises can help you to notice if one side is weaker or stronger than the other, so you can work on balancing things out.
It's kind of like when you're playing with blocks. If you stack all the blocks on one side of your tower, it might fall over because it's not balanced. But if you put the same number of blocks on both sides, your tower will stay strong and steady. When you do unilateral training, you're making sure that both sides of your body are equally strong (like how you need to make sure both sides of your block tower have the same number of blocks) so you can stay healthy and strong.