It means that if needed, you can always downgrade to 4.0.1, say when Apple sends out 4.1 and won't let you downgrade any longer, you can still downgrade if your SHSH is on file for 4.0.1.
It means that if needed, you can always downgrade to 4.0.1, say when Apple sends out 4.1 and won't let you downgrade any longer, you can still downgrade if your SHSH is on file for 4.0.1.