From the wiki on Securing Your wallet:
"Backup all data (500MB)
Follow these instructions to backup all the bitcoin data (wallet and block chains) to an encrypted disk image.
1.Open Disk Utility
2.Click New Image and choose 500MB, 128-bit or 256-bit (faster or more secure) encryption and single partition.
3. Save it somewhere you won't lose it (like your Dropbox, Strongspace or whatever)
4. Choose a safe and strong password
5. Move everything from ~/Library/Application Support/Bitcoin/ to the image
6. Symlink it back so the app would be able to use it
ln -s /Volumes/Bitcoin ~/Library/Application Support/Bitcoin
Don't forget to mount your image before using Bitcoin and unmount after quitting it.
Backup just wallet.dat (40MB)
Follow these instructions to backup just the wallet.dat file. This results in a smaller disk image, but it's more complicated to do.
1. Open Disk Utility
2. Click New Image and choose 40MB, 128-bit or 256-bit (faster or more secure) encryption and single partition.
3. Save it somewhere you won't lose it (like your Dropbox, Strongspace or whatever)
4. Choose a safe and strong password
5. Move your wallet.dat file to the image
Symlink it back so the app would be able to use it
ln -s /Volumes/Bitcoin/wallet.dat ~/Library/Application Support/Bitcoin/wallet.dat
Can someone elaborate even a little bit on these steps. What is "symlink"?