By allowing other stations to relay your stream, you are allowing a remote streaming server to use your stream as the source for their broadcast, and they will rebroadcasting your stream as their own.
The procedure for allowing relaying of your stream varies depending on the streaming server software you are using.
To permit relaying:
In the navigation panel at the left-hand side of the screen, click Settings under the Configuration heading.
Click the Relaying tab. If you do not see a Relaying tab, then you are not using a SHOUTcast server and are looking at the wrong section.
In the Allow other servers to relay stream field, select Yes.
In the Allow other servers to make stream public field, select Yes if you want to allow other stations to advertise their rebroadcast of your stream in public station listings.
Click Update to save your changes.
Stop, then restart the server to apply your configuration changes.
When your server comes back online, the remote server may be configured to relay your stream.
No steps are required to permit relaying under IceCast.
Logically there is no reason to include an option (as provided by SHOUTcast) to enable or disable relaying, as anyone who can tune into your stream as a listener could (with a bit of expertise and the right tools) fairly easily "rip" your stream and rebroadcast it without permission anyway. The option to disable this relaying under SHOUTcast only makes it marginally less convenient to do so.