TPU is an elastic, thermoplastic (polyurethane) plastic.
There are now various elastic plastics in the field of 3D printing, the most well-known of which are by far TPE and TPU filaments. These thermoplastics can change shape, meaning they can shrink or expand as long as some pressure is applied to the material.
TPU is EXTREMELY deformable compared to other filaments, it has very good thermal properties and dimensional stability up to approx. 95°C.