Answer:
spam, bulk email, unsolicited email. Same as the junk mail your mailman leaves in your mailbox. Over time Spam also has added malicious code. in other words TRASH that takes up all your Email space with commercials. Most of the time these commercials are completely useless. After time you can get pretty annoyed by this kind of trash. Also it's not always the commercials, sometimes its the other useless information that you probably don't need.