The functional requirements specify what the product should do. They are related to the actions that the product must carry out to satisfy the fundamental reasons of its existence. Think of the functional requirements as the business requirements. That is, if you speak with a user or one of the business people, they will describe the things that the product must do to complete a part of their work. Keep in mind that the requirements specification will become a contract for the product to be built. Therefore the functional requirements must fully describe the actions that the desired product can perform.
Non-functional requirements are the properties that your product must have. Think of these properties as the characteristics or qualities that make the product attractive, usable, fast or reliable. These properties are not necessary because they are fundamental activities of the product (activities such as calculations, data manipulation, etc.), but they are there because the client wants the fundamental activities to work in a certain way. They are not part of the fundamental reason for the existence of the product, but they are needed for the product to work in the desired way.
Non-functional requirements do not alter the functionality of the product. That is to say, the functional requirements remain the same regardless of the properties attributed to them. Non-functional requirements add functionality to the product; It requires a certain amount of pressure to make the product easy to use, secure or interactive. However, the reason why this functionality is part of the product is to give you the desired characteristics. Therefore, you can think of the functional requirements as those that do the work and the non-functional requirements as those that give character to the work.
Non-functional requirements constitute an important part of the specification. They are important since the client and the user can judge the product in its non-functional properties. As long as the product meets the required amount of functionality, the non-functional properties - how usable, convenient, attractive and safe it is - can be the difference between an accepted, popular and an unused product.