It means literally that the decryption operation of the wallet failed.
If the password you enter is different in any way from the password you entered when you encrypted your wallet with "File | Add Password" you will get this error.
I would check things like:
Are you definitely entering the same password you used to encrypt this wallet.
CAPS lock key
If you use an Input Method Editor, are you entering exactly the same characters.
There is also another copy of your private keys on your disk if you need to use them:
When MultiBit encrypts the wallet it writes out the private keys to an export file, encrypted with the wallet password. As long as you have the correct password you can create a new wallet and import the private keys into the new wallet. You will then have access to your bitcoin.
There are more details about this in the help (https://multibit.org/en/help/v0.5/help_contents.html) in the topics:
- Help on specific tabs | Importing private keys
- More technical details | File descriptions