Usually a system design process starts with understanding the system's purpose and one way to understand system's purpose or goal, is to clearly define a list of requirements.
These requirements allow us not only to understand how the system will be used and how it works, but also set clear boundaries which will make sure our design is focused on the right aspects of the system design. We usually distinguish between functional and non-functional requirements.
Functional requirements are used to specify an expected function or a behaviour of the system. Simply put, something the system should be able to do.
For example, for a video streaming service a requirement might be to upload a video or comment on a video. For instant messaging application, a functional requirement will be, to be able send and receive messages.
Non-functional requirements focus on how the system performs, especially in general and not focusing on specific functions.
While such requirements might affect user's experience they shouldn't affect specific functionality or features the system supports.
For example, if a system is a type of a service, a non-functional requirement might be "zero downtime" or "No loss of data".