"Not my bag" is an idiomatic expression that means something is not within one's area of interest, expertise, or responsibility. It implies that the person has no particular inclination or ability to deal with a certain matter.
For example, if someone is asked to solve a complex math problem and they respond with "not my bag," it indicates that they are not good at math or have no interest in solving such problems.
This phrase can also be used in a social context. For instance, if a group of friends is discussing a particular hobby that one person is not familiar with or has no interest in, they might say "that's not my bag."
In conclusion, "not my bag" is a useful and commonly used expression that helps to convey one's lack of interest or ability in a particular area. It allows people to be honest about their limitations and avoid getting involved in things that they are not comfortable with.