Md5 Hash can’t normaly be decoded. But there are tools that can decode just a few MD5-Hashs.
MD5 Hashs are used for saving keys in databases. So if a hacker gets your database, he can’t do something with the keywords.
The decoder tools just encode Strings and save them in their database. If you are lucky, they have the encoded string for your md5 Hash. A tool like this is MD5 Decrypter.
How you can hold your passwords save
You can add some text bevor and behind the undecoded password in your website. So the hacker can’t get the original password.
For secure passwords you should just allow passwords with special characters.