MD5 is the name of a hashing algorithm. It's most often used to hash a string like "myl33torpassword!", which becomes "4bc8c76eb067703858bc7c90de9eb6ad". It's one way, so you can't derive the original string from the hash; all these "cracks" are just brute-forces or a rainbow table (as I understand it) is essentially just a table of precalculated hashes.
The cool thing about MD5 (or all hashing algorithms) is that the hash varies wildly depending on it's input - so as a hacker you can't say "ooh I'm getting close"...
"myl33torpassword!" => "4bc8c76eb067703858bc7c90de9eb6ad"
"Myl33torpassword!" => "1c566b8dff7c7c8558c723d3288a8dcc"
So, for that reason it's also used widely to checksum files. You can tell if a file's corrupted, or more seriously, if it's been altered from the original.
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iv tryed everything!!!!!!!!!! could u please just add The gun and shooting Code thats All!!!!!!!!!