APN protocol / APN roaming protocol
Android devices which support both IPv4 and IPv6 networks will have entries here for APN protocol and APN roaming protocol. These specify whether your device should enable IPv4, IPv6 or both on your device.
IPv4 is required to access most of the Internet, and this is unlikely to change in the medium term, but if your carrier supports IPv6 as well, then you can enable it as well. IPv6 support will be more useful in the future if and when more sites or other services begin to adopt it.
You will gain the ability to access sites which only have an IPv6 address (very few such sites at the moment) and may also benefit from more efficient access to sites which support both.
If your carrier does not support IPv6 and you enable it, then it will probably just have no effect and you will continue to use IPv4 as normal, except in some rare cases where your carrier's network is mis-configured.
What is IPv4 and IPv6?
For most of the Internet's life the fundamental protocol underlying Internet communication has been IPv4. This protocol allows sending packets from one machine to another over the Internet based on numerical addresses for each machine on the Internet.
IPv4 allows around 4 billion unique addresses, and are assigned by the Internet Assigned Numbers Authority (IANA) to ISPs in various countries according to need. Unfortunately, the world has already allocated most of the 4 billion possible addresses, and action is needed to ensure the Internet can grow beyond any limits this may impose.
IPv6 addresses this by using a much longer unique address, 128 bits long instead of 32 bits. This increases the number of possible addresses world-wide from around 4 billion to 340 billion billion billion - way more than the number of atoms on our planet and enough to give each machine on the Internet not one, or a small number, of addresses, but potentially an address space larger than the entire number of IPv4 addresses.