Detail or dressing is the collection of people, animals, furniture, shrubberies , etc that take a plain streetscape, moonscape, forest or other landscape and turn it into an entertaining scene. It can be compared to greebling but on a larger scale.
Adding life to static scenes can take a very long time. For example, large Classic Castle City layouts often require hundreds of medieval soldiers to be set up individually. Many builders take great pride in this tedious work and create action and humour with minifigs, animals (often sheep) and a few small props.