1. What does it mean?
쌤 is a colloquial contraction associated with 선생님. You may hear it with a subject or name, as in 수학 쌤, or as a direct address to a familiar teacher or instructor.
Also written or said as ssaem
2. The nuance
It can sound friendly, but the acceptable level of familiarity depends on the person. 선생님 is safer in an introduction or formal message; 샘 is another informal spelling.
3. Examples
“기타 쌤이 손가락에 힘을 빼 보라고 하셨어.”
My guitar teacher told me to relax my fingers.
“쌤, 지난번에 빌린 책 돌려드리러 왔어요.”
Teacher, I've come to return the book I borrowed last time.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.