Zen is a complex set of teachings in Buddhism that emphasizes the tangible, everyday nature of enlightenment as opposed to a more intellectual, scholarly approach.
HOWEVER, in English it is used to simply mean "not bothered/stressed; relaxed". It's often used to imply not being stressed as a result of being more intelligent or patient. For example:
"I have to spend Christmas with my mom. Her and I never get along but I have a very zen attitude about it now."