A payload is the piece of software that lets you control a computer system after it’s been exploited. The payload is typically attached to and delivered by the exploit. An exploit that carries the payload in its backpack when it breaks into the system and then leaves the backpack there.
Metasploit’s most popular payload is called Meterpreter, which enables you to do all sorts of funky stuff on the target system.
For example, you can upload and download files from the system, take screenshots, and collect password hashes. You can even take over the screen, mouse, and keyboard to fully control the computer.