Mechanical, electrical and plumbing (MEP) refers to these aspects of building design and construction.
MEP design is important for planning, decision making, accurate documentation, performance and cost-estimation, construction, and operating/maintaining the resulting facilities.
MEP specifically encompasses the in-depth design and selection of these systems, as opposed to a tradesperson simply installing equipment.
Many Architect / Design consultant including the client focus more on aesthetics rather than on MEP design. However MEP is the backbone for any project.