An MD5 hash basically converts a text into a 32 character series of text and numbers (no matter how long the original text was).

This is typically used in databases so passwords aren't saved in plaintext. For instance your username 'monkeytorture' put through an MD5 hash turns out to be: d10557ecf7f78024d392a9d2a5dcf11b

Also remember that this hash is not at all random and completely depends on what the original text was, if you MD5 hash the text 'monkeytorture' it will always be: d10557ecf7f78024d392a9d2a5dcf11b

Now keep in mind that this MD5 hash is not reversible, that meaning if you managed to stumble upon the aforementioned MD5 string you couldn't deconstruct it back to it's original form. But what you can do is compare two MD5 hashes and if they are the same then you know whatever you hashed was the correct.

What dustball did was put the MD5 string, the problem being that nobody could deconstruct it. You would essentially have to try every combination of characters hash it and compare it against the provided string until you found the match. Now what dustball was talking about with rednoize is that it's basically a site with a huge database of MD5 hashes and their original strings, you type in your MD5 hash and if it has a match it will tell you the original string. But again until somebody types in 'monkeytorture' it won't know what 'd10557ecf7f78024d392a9d2a5dcf11b' was originally.

After this picture got insanely popular people who realized this was a 32 character string realized it was likely an MD5 hash, but people who typed in the MD5 hash wouldn't be able to come up with the original text until dustball inputted said text to rednoize. As soon as he did that he just had to wait for the next person to try rednoize and voila, they know what the original text was.

Here is a site that will convert whatever string you want into and MD5 hash.. Try typing in your username and generate the hash.

Now copy that hash over to Rednoize. If your username is obscure it is likely that nobody has ever hashed that text and the results will show 'hash not found.' But then type in your username into Rednoize this time, and it will tell you what the hash is, but now the website knows what your username is in it's hashed form if you copy and paste that hashed text into Rednoize again the site will then be able to tell you what the original text was.

I hope this clears up at least a little bit.