Spanish words that contain -gue or -gui are pronounced by omitting the u sound and making a hard g.
But there are a few words in Spanish containing -gue or -gui that are pronounced differently: the sound of the u is not omitted and the g is still hard. To indicate this, the written word uses the orthographic symbol called a "diéresis". This symbol looks like two dots above the letter u. Some examples using the dieresis are:
Notice how the pronunciation varies if we do not include the dieresis:
U is the only Spanish letter that can have a dieresis