The most important setting in the group, APN stands for Access Point Name. Whenever your phone 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 is a short string of letters and other characters intended to identify a particular connection class.
The APN you configure must match one of the APNs your carrier accepts, and this varies by carrier, so you should ask them (or search for their recommended APN settings on their website) 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.