What does Islam delenda est means?

It means Islam must be Destroyed.

It is a take off of the ancient Roman expression: "Cathago delende est." Or: "Carthage must be destroyed.", spoken over and over by Cato, warning against coexistence with a predatory rival.

The Word Islam means "Submission", and it comes from the Arabic Word 'ASLAMA' which means subservience, obeisance, or surrender to GOD.