Pre-Socratic philosophers were Greek philosophers who lived in the sixth and fifth centuries BCE and developed the theories of material monism. Material monism is the concept all of the world’s objects are formed from one single element. The reason that this is important in the development of philosophy and science is that these philosophers were the first to provide an explanation of the world around them without attributing events to supernatural events. They are viewed as being the first scientific thinkers and philosophers within Western culture and tradition. When we look at this in the concept of a timeline, by definition these philosophers worked before Socrates (one of the most famous philosophers). The problem with looking at this concept chronologically however is that many of the later Pre-Socratic philosophers where actually his contemporaries who learned from the same philosophers as he did (the Pre-Socratic philosophers).
In addition to looking at this term as a timeline for the development of philosophical thought some believe that the term Pre-Socratic also lies within the quality of the theory developed by the philosopher, meaning that the theories developed by the Pre-Socratic philosopher would be inferior or lesser than the theory developed by Socrates, his contemporaries, or his predecessors.