Availability is an integral part of the Katch experience. Katch uses what is known as a Beacon to signal your availability.
When you make yourself available, you are "Free" to take incoming calls. If you turn off you're availability, you are considered "Busy" and you can connect with others the next time you become available.
You can also be available only for a certain batch. Just go into the batch, and activate your Katch Beacon.