A way to assign a destination address to any device connected to the Internet. Most common is the 4th version of that idea - called IPv4. Each device (node) ad 4 bytes long address assigned. This in theory allows 4,294,967,295 addresses. In practice address is looking like that: byte1.byte2.byre3.byte4 -> 192.168.2.2 using decimal notation.
This big group of addresses gets divided in smaller groups which are being assigned to many organizations to use for their devices. Division happens by creating groups fixing bytes on the left side of address. 10.x.x.x, belongs to one entity and 20.x.x.x belongs to another.
Recently the 4 billion addresses became not enough so IPv6 was invented.