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 much of the Internet, and should not be disabled. This is likely to be the case for many years to come. If your carrier supports IPv6 as well, then you can try enabling both. IPv6 support will be more useful in the future if and when more sites or other services begin to adopt it.
If you experience any problems accessing particular sites while IPv6 is enabled, it could be an indication that your carrier's IPv6 network is mis-configured or has a problem, and you can try setting this to IPv4 only.
If your carrier supports it, enabling IPv6 will give you 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 the most likely outcome is that enabling it will have no effect and you will continue to use IPv4 as normal, except in the rare cases mentioned above when a carrier's network is mis-configured.
What are IPv4 and IPv6?
For most of the Internet's life the fundamental protocol underlying Internet communication has been IPv4. This protocol allows sending packets (short pieces of information) from one machine to another over a global network (the Internet) using a unique numerical address given to every machine on the network.
The length of an address in IPv4 allows for around 4 billion unique addresses, which 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 was created as a new Internet protocol to address the limits imposed by IPv4. It uses 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 in the human body and enough to give every subscriber to an ISP in the world not just one address but an address space larger than the full number of IPv4 addresses.
IPv6 is a separate and incompatible protocol to IPv4 so those using it will need to keep IPv4 enabled as well in order to reach any sites that are not IPv6-enabled.