Let's cover the definition as it applies to email one part at a time. If you suspect this might have happened, simply resend it. Anything from someone in your own home or office looking in your drafts or sent items folder, a malicious internet operator peeking at traffic as it passes through their systems as your email gets routed the the destination server or lastly the wrong person sitting in front of the recipients computer and snooping their email. So if the message contains sensitive information, investigate one of the methods to secure your email. Although that last is misleading.
Plain text is readable by almost anyone, anywhere on the Internet.
HTML - the same 'programming' language that is used to display pages in your web browser, but used here instead to allow you to add much more personalization and style to your emails. Bold, italics, underlined fonts. Bigger, smaller, or different colored fonts. In truth, it is no more difficult to use then the formatting features of your favorite word processor. But older systems or the 'toys' might not be able to.