RAM is used to store data on the blockchain, measured in bytes. It has to be bought with WAX.
CPU is the time your transaction needs to be completed, measured in microseconds.
NET is how big your transaction is, measured in bytes.
CPU and NET go hand-in-hand when processing transactions. If there are insufficient amounts available to use, the transactions will fail.
For more information about RAM/CPU/NET you can read this blog post: https://medium.com/wax-io/what-is-net-cpu-and-ram-on-the-wax-blockchain-e17d2ff9855e
You can also watch this helpful video about resources: https://www.youtube.com/watch?v=GAts5Pwg_Is