Imagine you have a stick, and you want to bend it into a circle. The bend radius is how bendy the stick is without breaking or cracking. So, if the stick is very bendy, you can make a smaller circle without it snapping. But if the stick is not very bendy, you can only make a bigger circle before it breaks.
Bend radius is important when dealing with things like wires or pipes, which are often bent into different shapes. If you bend them too much or too sharply, they can break, which is bad news. So, it's important to know the bend radius when working with anything that needs to be bent.