If you use iMessage frequently for both work and personal, there may come times when you don't want to be disturbed by messages from people you don't know. iMessage gives you the ability to disable alerts from people that aren't in your Contacts app. You'll still receive the messages, you just won't receive audible alerts for messages from people you don't know.
That's it. You'll now only receive alerts for messages from people you have in your contacts. It is worth noting though that this feature only works for iMessage alerts. If someone not using iMessage sends you a regular text, you'll still receive it and get an alert.
If you're having issues getting iMessage to activate, get more help at the iMore forums, and for more tips, check out: