It is a phrase. It means to agitate or stir, but usually in a personal way. As in - to say things that will generate discussion or action from other people.

In the US "Shake things up" can come with implied conflict or force: The things getting "shaken up" are not inviting change and/or do not want change. Typically this happens in business or social settings. In sports, "shake things up" often means that important personnel changes have been made. These changes are usually controversial in some way. In business, "shake things up" often means that a new manager or leader is changing the way things have been done. Usually in an effort to save the company from failure. Socially, "shake things up" can mean different meanings. 1. Change the current social order for the better. (End some form of discrimination). 2. Do something dramatic to draw attention to yourself. (Show up naked to a formal event).