An APN (Access Point Name) is the name for the settings your device reads to set up a connection to the gateway between the carrier network and another network, most frequently the public internet.
The carrier network then reads these settings, determines the correct activation IP address, connects to the correct secure gateway and finally checks if you need a private APN.
The APN must be correctly configured in accordance with the carrier requirements otherwise the connection will fail.
One area where we see regular failure is when the incorrect APN is input via your hardware.
You can find a comprehensive list of carriers and APNs supported by Concirrus here.