An enterprise, or cloud application, solves business problems and other real-world problems. These problems cannot be resolved without knowledge of the particular domain. For example, you cannot provide a software solution for a financial system such as online stock trading if you don't understand stock exchanges and how they function. Therefore, having domain knowledge is a must for solving problems. Now, if you want to offer a solution such as software or an application, you need to have some domain knowledge to design it. Combining the domain and software design is a software design methodology known as DDD.
When we develop software to implement real-world scenarios offering the functionalities needed for a domain, we create a model of that domain. A model is an abstraction, or a blueprint, of the domain.