Okay, kiddo, so you know that when you push or pull something, you're using force, right? Well, a pound-force is just a way to measure how strong that force is.
Imagine you're playing with a stack of blocks. If you use your hand to push the stack, you're using a force. That force is like a measurement of how hard you're pushing. If you push with the same strength as one pound, that's called one pound-force.
Now, pound-force is used a lot when we talk about weight and gravity. See, when you step on a scale to weigh yourself, you're actually measuring the force of gravity pulling you down. If you weigh 100 pounds (which is a measure of how heavy you are), that means that the force of gravity on you is equal to 100 pound-force.
So pound-force is just a way to talk about how strong a force is. Whether you're pushing a stack of blocks or measuring the weight of an object, pound-force helps us understand how much force is being used.