A plasma chain is one or more separate (child) chains anchored to the main (parent) chain.
A tree-like structure can be used to create an unlimited stack of child chains, so work can be taken away from the main chain to increase bandwidth. These child chains periodically report back to the main chain and use it to settle any disputes (similar to optimistic rollups).
Plasma chains are similar to side chains but are non-custodial and do not introduce a new consensus mechanism.
As child chains can be optimized for specific use cases and run in parallel to each other, they can reduce cost, improve speed, and have specialized features.
Plasma was one of the earlier areas of scaling research but failed to gain traction as it could only handle basic transactions, had delayed withdrawals, and relied on operators to watch the network and store the data.
Examples: OMG Network.
Plasma explanation by Ethereum: https://docs.ethhub.io/ethereum-roadmap/layer-2-scaling/plasma/