If you start to learn anything about the internet, then you will start to realize that it is all about addresses. Addresses that are not too dissimilar with how your house works with the post office. The post office gets a letter with your name and address on it and then they know where to send it. That is the same way that the internet works. They see your internet address and the server knows where to send the data packets to.
This address metaphor has a long history when it comes to computer science. You probably do not know it but the memory, otherwise known as RAM, in your computer works with the same address metaphor as well. Before the data is brought to the CPU to be processed, it must go through the memory first. In the memory each piece of data is stored as what is known as a memory address. So as you can see, addresses are used a lot when it comes to computers.
In this day and age, with the internet, we use a technology that is known as IPv4. The IP stands for internet protocol and it is the type of data that is sent through a network. The problem with the IPv4 protocol is that we are running out of addresses. Pretty soon there will be no more IPv4 address to give to people. Everything that is able to connect to the internet must have an IPv4 address or they will be invisible on the internet. The solution that is being implemented is called IPv6.
IPv6 is pretty much the same thing as IPv4 except that it allows for a longer array of web addresses to be assigned. While the amount of addresses to be assigned is not infinite the number is so large that it seems to be.
The problem now is that not enough industry people are switching to IPv6. So to help that along they made up a day to try to raise awareness about the issue. While the day has gone by with little fanfare there are some large companies like Google and Facebook who recognized it. That is a good start.