Says <anything> out loud, so that everyone in the same room hears it. This is so commonly used that there's a special abbreviation for it: any command-line beginning with a double-quote ('"') is treated as a 'say' command.
Example:
Munchkin types this:
"This is a great MOO!
Munchkin sees this:
You say, "This is a great MOO!"
Others in the same room see this:
Munchkin says, "This is a great MOO!"