As you are probably aware, while an adequate conclusion may simply sum up the information already presented, an excellent conclusion will apply the thesis of the essay to a larger context or consider its wider implications, elegantly and persuasively demonstrating its validity. Therefore, although I do not know what perspective you have adopted in your examination of the Progressive Era, I can suggest some ideas which might be helpful in framing your conclusion.
There is some debate as to the exact dates and duration of the Progressive Era. Most historians regard it as having begun in the mid 1890s, succeeding the Gilded Age (and being, to some extent, a reaction against the excesses of that time). However, opinion is divided as to whether the Progressive Era can be said to have ended with the First World War or more than a decade later with the onset of the Great Depression. You will presumably have taken a position on this in your essay and can use the conclusion to explain the principles behind your thinking. What aspects of the Progressivist philosophy were applied in the politics of the Progressive Era? How did the War or the Depression alter the situation? This approach will focus your conclusion on the connection between philosophy and politics.
You could also think about Progressivist philosophy in a more abstract sense and consider whether and how it has been influential after the Progressive Era. You could consider recent movements such as Black Lives Matter, Occupy Wall Street, or the campaigns for a minimum wage and universal health care, linking their guiding principles to the trustbusting, labor reforms, and women's suffrage movements of the Progressive Era.