The tools and protocols discussed in this article all have one thing in common: they all send data over the Internet in plain text. What this means is, when the data is leaving one user's computer on the way to another computer, it's not encoded in any way.
Imagine someone having a phone conversation with someone in another city. Then, in the first person's house, someone picks up an extension and hears the conversation. This person can now eavesdrop on the conversation without difficulty. This is exactly what can happen to network traffic as it travels between computers.