The "Internet Protocol" (IP) address is like a phone number for every device on the internet.
The addressing system in IPv4 was made up of four 8-bit (0-255) numbers, so an IPv4 address looks like something like 192.168.1.1.
IPv4 is limited because it can only provide ~4.3 billion unique addresses and the growth of the internet means the number of available addresses is running out.