All internet connectivity online requires an IP address (such as 126.96.36.199), a numerical representation of the origin and destination of data transfers. When originally invented, the IPv4 address system could accommodate about 4.3 billion different IPs. This was more than enough as at that time, the internet was tiny and only a few people were even able to use it. Decades later though and those 4.3 billion IP addresses are nowhere near enough for the estimated 17 billion internet-connected devices around the world.
While the IPv4 system has been improved to be more efficient, there are still simply not enough IPv4 addresses to go around for everyone and this is what IPv6 is meant to solve.
IPv6 was created to replace the 32 bit IPv4 addresses (4.3 billion combinations) with 128 bit IPv6 addresses which in total can facilitate 340 billion billion billion billion devices. Or more precisely, there are 340,282,366,920,938,463,463,374,607,431,768,211,456 different IPv6 address combinations. It will be a while before we run out of these.
However, you might wonder why IPv4 is still in use if there is a better system. That is because of how engrained IPv4 is in our internet infrastructure. The internet as whole is immensely complex and have millions of moving parts to facilitate everything correctly. The routers we use, the internet providers we sign up for, the communication protocols and much, much more have been built for decades around IPv4 connectivity.
This means that to fully switch over to IPv6, every single one of those millions and billions of parts needs to be able to use IPv6 or the device simply be able to get online. For a long time now, very smart people have been slowly transitioning everything they can to be compatible with IPv6, any many things have operated on IPv6 for a long time now, but we are still not quite at the point where retiring IPv4 is feasible.