In a shared channel, multiple slaves synchronize to the same master (i.e., share a channel). Depending on the size of the addressing, you can have up to 255/65535 slaves.
You can find a small intro to shared channels here http://www.thisisant.com/developer/resources/tech-bulletin/demystifying-ant-topologies/
Shared channels are described in more detail in Section 5.6 of the "ANT Message Protocol and Usage" document, and the application note AN07 - Auto Shared Channels.
