Astar (previously Plasm) is a Substrate Runtime Module Library which allows developers to add Plasma functions to their Substrate chain. By adding a Astar (previously Plasm) Substrate Runtime Module Library, you can get scalable blockchains within a few minutes.
Some people might not know Substrate Runtime Module Library. Basically speaking, Substrate consists of 2 components, Substrate Core and Substrate Runtime Module Library aka SRML. We can customize Substrate Core with SRML and make an original Substrate chain. Other people might not know Plasma. Plasma is a layer2 scaling solution which makes it possible for scalable computation by structuring economic incentives to operate the blockchain autonomously without the operator’s management. Ideally, it brings infinite scalability into your blockchain.