ZRX is a protocol based on the Ethereum blockchain that enables for the decentralised and low-cost trading of ERC 20/721 tokens. Most individuals now use a centralised exchange to redeem these tokens, which makes the process more complicated. Relayers are responsible for spreading pending orders through private or public order books in the 0x system. As a centralised exchange would, this allows it to offer liquidity to the network by hosting order books.
A relay, unlike a centralised exchange, cannot execute orders. It can only make operations easier by bringing in takers to the network.