Definition:
Model-driven architecture (MDA) is a type of approach to software design, development and implementation. As the name suggests, this approach uses models as a set of guidelines used in structuring design specifications. The reason why models were selected as the central object in this design principle is because it helps designers to reason out the system design by allowing them to ignore extra details and giving them more focus on the relevant issues. Models are also used all throughout the engineering and design field to understand complex and real-world systems.
Further Reading:
Book: UML Distilled Third Edition by Martin Fowler