The tréma is an accent that is only used on 2 vowels in French: ë and ï. Its purpose is to let readers know that that vowel should be pronounced separately from the vowel that comes before it. So basically, this accent prevents two vowels from being pronounced together.
An example of a tréma is in the French word for corn: maïs. Another example is in the French word for canoe: canoë (it's spelled exactly like the english word except for the tréma on the e).
