Within EOSIO, a user can delegate their resources (CPU Bandwidth and Network Bandwidth) to another user. To incentivize this, a borrower can pay rent towards the lender. REX, a system level resource exchange smart contract written by block.one, is a tool that will decentralize this entirely. Rather than paying interest directly from a borrower to a lender, REX creates a pool based on a Bancor relay to distribute all collected interest between all lenders.
REX also includes a caveat that a user must be either voting for at least 21 Block Producers or be delegating their vote towards a proxy to be eligible to utilize the REX.