What do 2 dots above a vowel mean?

In french, the combination of certain vowels changes the phonetic of them. Clear examples are O and U that together are pronounced [u] or A and U making and O sound together. But in some cases you might want both vowels to sound independently while being next to each other. In such case there are 2 dots written above the second vowel. These dots are called "tréma". Examples can be words such as the name "Saül" or more commonly above the letter I in words such as "maïs", "naïf" or "héroïne".