Okay, so imagine you have a very long string of beads, all strung together. That's kind of what a polymer is like. But if you pull on that string really, really hard, it might break. That's called a fracture.
Now, polymers can also fracture if they have weak spots in the string of beads. These weak spots might be caused by things like tiny bubbles or impurities in the polymer.
And sometimes, when a polymer gets too hot or too cold, it can become more brittle and prone to fracturing. So we have to be really careful how we use and store polymers so they don't break.