Dandere, oftenly confused with kuudere, is a common anime trope used to describe people who are often quiet and extremely shy towards people they like. Despite this, once they do start to conversate (usually awkwardly) with the one they admire, they become more open and less shy.
The reason this is confused with the definition kuudere is because both tropes hardly talk, and occasionally show their more open side. A kuudere is someone who is often emotionless and clueless, but will sometimes show their loving and kind/sympathetic side.

A character that is a dandere: Hinata Hyuga from Naruto.
A character that is a kuudere, not a dandere: Kanade Tachibana from Angel Beats!

A dandere is a character archtype that entails someone being quiet which is usually associated with shyness. "Dan" comes from the word "danmari" which means silent and taciturn. "Dere" means to become "lovey dovey"

Not to be confused with Kuudere which is a cool person that becomes lovey dovey. They are very similar in appearence and possibly behavior, but their underlying character reasoning is different. Being silent for the sake of being silent vs being cool