When you make a toy out of tiny pieces that fit together like LEGOs, it's important to make sure each piece is the right shape and size so it all fits together and works properly. Imagine that the toy is made out of a material called plastic, which is a type of substance called a polymer. Polymer characterization is when scientists try to figure out exactly what the plastic is made of and how the little pieces fit together.
To do this, they might use tools like microscopes to look really closely at the plastic and see what it looks like up close. They might also use machines that can measure things like how strong the plastic is or how stretchy it is. They might even cut the plastic into tiny pieces and study them to see if they can figure out how it was put together.
All of these things help scientists learn more about the plastic, so they can make even better toys (or other things) in the future. By understanding how the polymer is put together, scientists can figure out how to make it stronger or softer, or maybe even make it change shape when it gets hot or cold.