That depends on the type of plastic.
Consider a ball pool for children.
It is easy to bath in it, because its easy to move them away. So it’s like a liquid.
If you would now glue two balls together and make a new ball pool out of these, is would get harder to swim, as the two balls are not round anymore.
Now imagine a chain of several thousand balls.
If you move one, you also have to move a lot of other chains. So they basically become solid.
Polypropylene and Polyethylene are such chains of carbon molecules.
With chemical reactions small molecules (propylene and ethylene) are glued together to form long chains. This is called POLYmerization.
Hence the name polymer.