Spam is a nickname for unsolicited bulk email advertising a product or service. Spam wastes people's time, and eats up network bandwidth. This is a problem that has been growing steadily in recent years.
There is some debate about the source of the term, but the generally accepted version is that it comes from the Monty Python song, "Spam spam spam spam, spam spam spam spam, lovely spam, wonderful spam". Like the song, spam is an endless repetition of worthless text.