Here are some examples:
* de facto: meaning "in fact" or "in reality"
* de jure: meaning "by law" or "legally"
* de novo: meaning "anew" or "from the beginning"
* de rigueur: meaning "required" or "essential"
* coup de grâce: meaning "the final blow"
The specific meaning of "de" will depend on the context of the phrase or word it's used in.