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!

Whenever making a congratulatory remark, this is what you say. Made popular by Kamala Harris when she and Joe Biden won the presidency.