Borrowed directly from French, merci means "thank you." Using it adds a bit of continental flair to everyday expressions of gratitude.
The word merci shares a linguistic history with the English word mercy. Both words descended from the Latin mercedem, meaning "reward, payment, or wages." In Old French, this concept evolved into a plea for grace, kindness, or pity (retained in English as mercy), as well as a polite acknowledgement of a favor received.