In terms of e-mail then plain text is just what it says.
The text will be plain, no colour, no fancy fonts, no special text size, no images etc.
HTML e-mail is like sending someone a web page. The e-mail can have colour, fonts, images, hyperlinks etc.
BUT, you can also hide viruses behind HTML e-mails, so many people do not like them and set their e-mail program to recieve e-mails in plain text only.
So even if you write and send a HTML e-mail the person getting it may not see it as you expected.