This is IRC's anti-command. At first glance it doesn't seem to do anything. Usage is /say thing. What /say does is say thing in the current channel. That's it. The exact same thing that happens if you had just typed thing. So, why is it there?

Well, think about it. What if you want to say something that begins with a /? What if you want, in channel, to say "/mode +b *!*@*", just to demonstrate to someone? That's right, you can't do it, because it will interpret that / as meaning you're trying to send a command. So if you type /say before the command you want to say in channel, and instead of being interpreted the IRC client knows to send it as text.

Most people don't bother with the five-keystroke bother of typing the /say command, and just begin with a . when they want to talk in channel, i.e. ./mode +b ... or whatever. The . blocks the IRC command interpreter from doing its magic.

Just for the record, if you want to be REALLY unnecessary, you can always use in place of /say the syntactical equivalent /msg #channel-you-are-in thing.