Musicology means the study of music by a scholar. A scholar is an academic person, often a professor or lecturer at a university. Someone who studies musicology is a Musicologist.
Musicologists study all kinds of music. They study the history of music and learn about all the composers and how they developed their ideas and learned from one another. They can do this by studying music scores, or by looking for historical documents which tell us about musicians from the past. Musicologists may also study the philosophy of music (thinking about what music means).
They write articles in musical journals or books on music. This can help us to know and understand more about the music we play or listen to.