To understand multi chain wallets you need to know what a single chain wallet is as well.
A single chain wallet only works on one blockchain. For example, a wallet may only support Bitcoin. In this case it is a Single Chain Wallet. A Multi Chain wallet, on the other hand, works on multiple chains. For example, a wallet could support Bitcoin, Ethereum, and Polygon. In this case you can view all your crypto assets in one place, instead of viewing it on multiple different wallets.