Spam is to email what junkmail is to snail-mail. It's just an internet nickname (definiton, sense 5). A person who sends out spam can do it very cheaply, because they are only dealing with electrons, and you don't have to pay postage. It's a very economical way to send millions and millions of unwanted advertisements all over the internet.
How spammers (people who send spam) do it is they either collect or buy huge lists of email addresses and have an automated program go through the list and send the advertisement email to each address.
One way (of several methods available) spammers build lists of addresses is to use another kind of program that is somewhat like a web browser. It looks through millions of web pages and wherever it finds an email address, it adds it to the list of addresses to be spammed.
To prevent any such address-collecting (see above) program from putting my address on a spam mailing list, I ruin the address by putting extra text into it. That way, when someone tries to send a spam to my email address, it doesn't know where to send it because I added the extra text to it. The program that collected the address isn't smart enough to take out the extra text and make it a good address again. People, on the other hand, at least people who know about spamblocking, can fairly easily see to remove the extra text and make it a good address again. An example:
my full email address is "meta4mix" plus an @ sign plus "crosswinds.net". "crosswinds.net" is the computer that all my emails get sent to, and where I tell my computer to look to get my mail. "meta4mix" is my user name on that computer. To ruin an address, I put garbage text into the name of the computer, making it something like "crosswinds.remove.this.for.non.spam.net". The junk part, obviously, is "remove.this.for.non.spam.", only advertisers trying to send spam there don't know that, because the programs they use to collect email addresses from websites are too dumb to take it out. So they try to send mail to a nonexistent computer named "crosswinds.remove.this.for.non.spam.net", and not "crosswinds.net". A response comes back to the spammer somewhat resembling what you get when you send a mail to my ruined address. Humans can see what the junk part is by reading the address, which spamming programs can't do, and delete it so it names the right computer again.
It should be noted (at least it seems logical, anyway) that some email collecting programs are smart enough to figure out how to fix spamblocked email addresses. Therefore, the effectiveness of such spamblocking measures is at best questionable. But mine serve other uses as well, such as humor and limited artistic expression, so I don't mind, I guess. Sigh. Stupid spammers.
Wow. Long winded, ain't I?