A MAC address is something like an electronic serial number. Whenever info is sent from your computer, the packet contains the MAC Address.
ARP (Address Resolution Protocol) resolves the MAC address to an IP Address. In a sense this identifies the calling computer connecting to your computer.
Think of it in terms of a car - the MAC is like a Vehicle Identification Number (VIN) and the license plate like an IP Address. You can easy change the license plate and pass it to someone else, in theory, you can’t do that easily with the VIN/MAC address.
RARP is simply the reverse of ARP. It effectively finds the MAC address of the device with an IP Address.
It’s important to note that network switches and routers can mask the correct MAC address of something as they can effectively replace it with their own as the packet travels through them