Linguistics is the scientific study of human language as it is used to express our thoughts, emotions, and desires. Linguists analyse these knowledge systems and consider how they are acquired, how they are used as tools to produce and understand our ideas, and how our interpretation of the way we phrase our ideas changes with time.
That linguist must themselves know how to speak in many languages is a misconception- linguists study Language as a concept rather than learn different languages. A linguistics degree, when combined with further study in other fields, can lead to work in a multitude of different industries from law to advertising to neuroscience.