a thermoset

Think of baking a cake or setting concrete: once heat and chemistry have done their work, the change is permanent. Reheating a baked cake does not turn it back into batter — it just burns. A thermoset is a polymer that behaves this way. During its curing it forms a dense network of covalent crosslinks tying all the chains into one giant connected molecule, and after that it is set for good. It will not soften and reflow no matter how you heat it; push the temperature far enough and it simply chars and degrades.

The mechanism is crosslinking taken to a high degree. As a thermoset cures — driven by heat, a hardener, or both — reactive chains bond to one another at many points, building a rigid three-dimensional network. Because the chains are now covalently tied together rather than merely resting side by side, they cannot slide past one another, so there is no melting. This is why thermosets are generally harder, stronger, stiffer, and far more resistant to heat and solvents than thermoplastics. Common thermosets include epoxies, phenolics (Bakelite), unsaturated polyesters, and vulcanised rubber (an elastomer is a lightly crosslinked special case of this family).

It matters because that permanent network is exactly what you want where a part must hold its shape and stiffness under heat and load — electrical insulators, circuit boards, adhesives, and the matrix of many composites are thermosets. But the same permanence is a real drawback: a thermoset cannot be remelted, reshaped, or easily recycled, because to reprocess it you would have to break the covalent network, which destroys the material. And there is a trade-off within the family: the more heavily you crosslink, the harder and stronger it gets, but also the more brittle.