a thermoplastic
Think of a candle or a bar of chocolate: warm it and it softens and flows, cool it and it hardens again, and you can repeat that cycle as often as you like to remould it into a new shape. A thermoplastic is a polymer that behaves this way. Its chains are separate, not tied together, so heating loosens their grip and lets them slide, and cooling lets them lock back into a solid. Most of the everyday plastics you handle — bags, bottles, tubs, toys — are thermoplastics.
The reason for the reversible softening lies in the bonding between chains. Within each chain the atoms are held by strong covalent bonds, but neighbouring chains are held to one another only by weak secondary forces — van der Waals attractions and, in some polymers, hydrogen bonds. Heat gives the chains enough thermal energy to overcome those weak inter-chain forces, so they slide past one another and the solid softens and flows. Cool it and the weak forces re-form and it re-solidifies. Nothing chemical is broken, so the change is reversible — that is exactly why thermoplastics can be melted and injection-moulded or blow-moulded into shape. Polyethylene, polypropylene, PVC, polystyrene, PET, and nylon are all thermoplastics.
It matters because reprocessability is what makes a plastic recyclable: melt it down, remould it, use it again. That is the great practical advantage of thermoplastics over thermosets, which are crosslinked and can never be remelted. One honest caveat, though: melt-reprocessing is not truly infinite. Each heating cycle can shorten the chains a little through thermal degradation (chain scission) or contamination, so recycled thermoplastic is usually a bit weaker than virgin material, and quality drifts down over repeated cycles.
A PET water bottle can be shredded, melted, and re-extruded into polyester fleece fibre — the chains never had crosslinks, so heat just lets them flow into a new form. A thermoset like a cured epoxy could never do this.
Thermoplastics soften and reflow on heating because chains are held only by weak forces — so they can be remoulded and recycled.
Reprocessable does not mean indestructible: each melt cycle can shorten chains (thermal degradation), so recycled thermoplastic is usually a little weaker. Contrast thermosets, which are crosslinked and cannot be remelted at all.