After your society has been prepared by the careful introduction of progressive policies, it starts what's known as the "scientific revolution." Of course, no guns or swords are involved here, but the revolutionary concepts and ideas which develop as a result of questioning centuries-old assumptions, such as that the Sun rotates around the (flat) Earth. After a number of these ridiculous concepts are conclusively disproved, people feel so enlightened that they are encouraged to seriously delve into science, along with other nations' scientists - which greatly benefits the nation's technological progress, of course.
This is logically and conceptually the last Policy to adopt in the Rationalism tree (although technically you can adopt it as third, right after Secularism and Sovereignty). The other two Policies, Humanism and especially Free Thought, are arguably much more useful, and more immediately so than Scientific Revolution.
The only exception to this rule would be if you have already entered into a number of research agreements and expect them to come to fruition soon. In this case, the immediate tech progress jump could be more useful than the long-term Science benefit.
It goes without saying that in order to make use of this Policy, you should from now on seek to enter into as many research agreements as possible; otherwise you won't see any effect. This naturally requires making friends with other nations, so a particularly bellicose civilization may have some problems benefiting from Scientific Revolution.
A scientific revolution is a period when rapid advances in human knowledge or technology overturns the current worldview, as a result triggering yet more advancements in thought and knowledge. Much of Europe underwent a scientific revolution in the 16th century, following the publication of Nicolaus Copernicus' work, "On the Revolutions of the Heavenly Spheres" and Andreas Vesalius' "On the Fabric of the Human Body In Seven Books." Both used modern (for the period) scientific practices to examine parts of the world around them and overturned incorrect scientific theories, some dating back to the ancient Greeks. Following the success of these books, the floodgates were opened, and scientists began carefully examining everything around them, and human knowledge increased exponentially over the succeeding centuries.