You should watch Split and Unbreakable before watching Glass in order to understand the significance of several of the relationships between some of the main characters. There is some explanation within the film as to who they are, so for factual purposes you do not necessarily need to have seen Split before Glass, but I think there is a high potential to miss some of this explanation and/or to misunderstand the importance of it unless you've seen the previous films. It was not presented as a stand alone film, rather it is very much a sequel of Unbreakable and Split.