Hi Paul,
When referring to resin in model making (car's ,boats ,planes,etc ) it's a 2 part liquid that's mixed.
One part is the resin and the other is the hardener.The mixture will harden quickly depending on the ratio and the ambient tempreture at the time of mixing. it should pour with a nice consistancy (like partially whipped cream)
It's used for small batch work and or upgrade items for existing kits, usually fairly costly to purchase due to limited run the nature of the market it's aimed at.
There are a variety of differences between using/building with resin which more experenced modelLers here may Identify for you ,as I've not built much myself. Wear a mask when sanding this stuff during clean up and preperation for paint as it can be toxic .
The "Plastic" used in industrial manufacture of model kits bares no resemblence to resin and is normally in granular form prior to entering the Molding machine,which in itself is a topic of it's own.