This problem is hard for a non-expert to understand.
Tips
memory
Literally 'a Han person outside the door' — someone who stands outside the door of knowledge and hasn't entered the field. 门 = door, 外 = outside, 汉 = man/person.
usage
门外汉 is informal and mildly self-deprecating. Use it to humbly admit you know little about a topic, e.g. '我是个门外汉' (I'm just a layman).
