Psycholinguistics, more simply put, is basically psychology of language. In this science, and subdiscipline of philosophy, we look at, and analyse,psychological and neurobiological factors that enable us as language speaking humans to acquire, apply and understand language.
There are many related disciplines and therefore research in the field of psycholinguistics is interdisciplinary. The subdisciplines include cognitive science, neuroscience, philosophy and psychology.
How does the brain process language? What is universal grammar? Do we have a language acquisition device? Or do we use cognition to acquire a language? if you were on Mars, how would you communicate? Do dolphins have a language? what constitutes a language? Is there a critical period for language acquisition? If so, who determines it?