The Materia Medica is a dictionary of homeopathic medicines and their symptoms. It is a book which is the final authority on homeopathy. The materia medica contains the list of symptoms experienced by provers of the medicine. The symptoms are arranged in a systematic order - Mind (symptoms related to mind/mental), Head, Eyes, etc..
It is not required for a doctor to memorise or remember all the contents of the Materia Medica. What is required is to understand the nature / keynotes of each remedy. A number of materia medicas have been authored. Prominent among them are Kent's Lectures, Hering's Guiding Symptoms, Allen's Keynotes, etc