The most important setting in the group, APN stands for Access Point Name. Whenever your device needs to open a data connection with your carrier, it quotes an Access Point Name. This is then used by the carrier to determine various settings on their end.
The Access Point Name itself is a short string of letters and other characters intended to identify a particular type of connection.
Traditionally, the Access Point Name used activated various filtering or translation of content at the carrier's side to ensure compatibility with older pre-smartphone devices. Now, most devices have fairly direct access to the standard internet and carriers tend to use fairly generic names for their APN.
The APN you configure must match one of the APNs your carrier accepts for your account type, and this varies by carrier, so you should try to look up their recommended APN settings on their website, or contact their technical support, to find out the correct name to put in this field.
If the APN does not match one that your carrier supports, or if it's left blank, this APN setting won't work at all.