From Coingecko:

Raydium is an automated market maker (AMM) and liquidity provider built on the Solana blockchain for the Serum decentralized exchange (DEX). Raydium has a first-mover advantage as an AMM within Serum and will be an integral part of bringing new and existing projects and protocols into the ecosystem. The protocol will act as a bridge for projects looking to expand to Solana and Serum, and in the process Raydium and the RAY token will become a foundation for enabling further development with partners, its own platform, and the ecosystem as a whole.

Unlike any other AMMs, Raydium provides on-chain liquidity to a central limit orderbook, meaning that Raydium LPs get access to the entire orderflow and liquidity of Serum.

Long term, Raydium aims to capture and maintain a leadership position among AMMs and liquidity providers on Serum, while leveraging the power of Solana to drive the evolution of decentralized finance (DeFi) and emerge as a leading protocol in the space alongside our partners and the community."