It uses smart contracts to create child chains and these child chains are in hierarchy. These child chains relay the information back to parent chain from time to time. As the child chains are created through smart contract, it has its own parameters & rules to govern the child chains. The transaction can also be reversed if there is a malicious actor in the child chain.
Any number of child chains can be created through smart contact in plasma concept. Therefore theoretically the number of child chains can be infinite, so the scaling capacity with plasma can be infinite too. Any child chain can further spawn it own child chain and so it can possibly form a branching which are rooted to the parent chain. So it is just like a hierarchical tree of side chains that periodically transfers information back to the main-chain.
These plasma chains not only provide scalability solutions but also lower the fee quite significantly.
There are so many other benefits of plasma-