The dieresis is a feature of Spanish language (and of many other languages) which consists on placing two horizontal dots as in /üe/ over the /u/ letter only in the /gue/ and /gui/ syllables to indicate that the /u/ letter must be pronounced, here are some examples:

Vergüenza

agüero

averigüéis

pedigüeño

jagüey

agüita

Güemes

ambigüedad

güiro

antigüedad

pingüino