What is the main chain in Polkadot?

Could you please elaborate on the concept of the main chain in the context of Polkadot? I'm curious to understand its significance within the broader architecture of this blockchain network. Is the main chain responsible for executing transactions? Or does it serve a different purpose? I'm also interested in knowing how it interacts with the various parachains and relay chains within the Polkadot ecosystem. Could you provide a high-level overview of its functionality and role in enabling cross-chain interoperability?