1. What does it mean?
츤데레 comes from Japanese character terminology. In Korean conversation it often describes a contrast between brusque words and thoughtful actions, or a character whose guarded behavior gives way to affection.
Also written or said as cheundere
2. The nuance
It is a playful description, not a reason to reinterpret rejection as secret interest. Cruel or controlling behavior is not made caring by calling it 츤데레.
3. Examples
“귀찮다고 하면서 우산을 챙겨 주는 캐릭터가 츤데레 같아.”
The character complains it's a hassle but still brings an umbrella for someone; very tsundere.
“말수가 적어도 다 챙겨 주는 친구를 츤데레라고 놀렸어.”
I teased my quiet but thoughtful friend by calling them tsundere.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.