Hey Lij. Are you looking for a YouTube video or...? What have you researched so far? What has made sense and what was confusing?

The basic idea is you put your EOS into a system-level smart contract and get back REX tokens. Then people pay into that smart contract to use EOS for CPU and bandwidth resources. The more EOS they pay into the contract, the more valuable your REX tokens become. Whenever you want to convert back to EOS, you sell your REX back into the smart contract and get EOS out (always more than you put in, but don't expect to make a lot of money until the demand for EOS resources goes way up). It uses the Bancor algorithm which is quite interesting and worth learning about (check out the great videos by EOS Weekly, I think they have one on Bancor).

This is the main interface I like most for REX: https://eosauthority.com/rex/statistics