The "clipboard" is nothing but an area in memory (inside your computer) set aside to temporarily hold data/content/information so you can transfer it from one place to another.
You never "see" the clipboard, which is one of the reasons why some people have a hard time understanding what it is.
The reason the clipboard exists is to make it easy to exchange data between programs. Without a clipboard, the only way to exchange data between programs would be to save the data to a file, and the load the file in the other program. Such a process would not even be possible if you want to transfer data between web pages.
So instead of saving the data to a file, you're saving it to an area of computer memory (called the "clipboard"), and then loading it from that area of memory into another page or program.
When you save something to the clipboard (the memory location in your PC), that's called "Copy" or "Cut". When you load something from the clipboard, that's called "Paste".