APN stands for Access Point Name, and is the name of the gateway between an IoT device and the internet. The APN must be configured in the IoT device.
A public APN is an APN that anyone can use. When an M2M simcard allows data use, it automatically connects to the public APN.
With a private APN, IoT devices are more securely connected to their own network. The private APN ensures that when an IoT device wants to reach the network, it is routed to its own network. Only IoT devices from one organization are on the private APN.