Every device connected to the Internet needs an unique IP address. IPv4 had been the standard used since the Internet started. However, the main problem with IPv4 is that we are running out of it. An IPv4 address takes the form of 255.255.255.255, a 32bit four octets representation. Each octet takes in a number from 0 to 256. Therefore, the total available IPv4 addresses are 2^32 addresses , slightly more than 4 billion of them. Of course, some address like 127.0.0.1 are reserved address and cannot be used for the internet.
4 billion addresses, that sounds like a lot. That is true 30 years ago, nobody would imagine the need to have 4 billion unique address. However, today, with the evolution of Internet, IoT, smartphones, smart tv, smart home, smart everything, we know that IPv4 will surely be not enough in the near future.
Introduction of IPv6 began, with a representation of eight groups of four hexadecimal digits, eg 2001:0db8:85a3:0000:0000:8a2e:0370:7334, bringing the total available unique address to 3,400,000,000,000,000,000,000,000,000,000,000,000,000. That is surely enough now.
Besides addressing the problem of scarcity, IPv6 also aims to improve security over IPv4. Example, the introduction of native IPsec support directly into the IPv6 protocol.
Other advantages of IPv6 includes
- Better multicast routing
- No NAT (network address translation) required
- Auto configuration
With IPv4 fast running out, all governments and organisations are encourage to begin supporting and migrating to IPv6. However, this is really not easy. There are alot of hurdles that is preventing the mass adoption of IPv6. Majority of Internet backend, especially in lesser developed countries, are still heavily dependant on IPv4. The complete transition from IPv4 to IPv6 will likely take decades to complete.
This answer references some points from https://en.wikipedia.org/wiki/IPv6_address