1. What does it mean?
짱 is an informal way to praise someone or something as excellent. It can stand alone as a reaction or appear in 짱이다, meaning that something is great or the best.
Also written or said as 짱이다, jjang
2. The nuance
This is enthusiastic everyday praise, not necessarily a literal first-place ranking. With a noun about a group, 짱 can also mean its leading or toughest person, so the surrounding sentence matters. Use more specific praise in formal writing.
3. Examples
“네가 만든 지도 덕분에 길을 바로 찾았어. 너 짱이다.”
Your map helped me find the place right away. You're the best.
“이 가방은 가벼워서 여행할 때 짱이야.”
This bag is so light; it's great for traveling.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.