Who was Theodore Roosevelt and what was the most important thing he did during the progressive era

Politicians and government officials. President Theodore Roosevelt was a leader of the Progressive movement, and he championed his "Square Deal" domestic policies, promising the average citizen fairness, breaking of trusts, regulation of railroads, and pure food and drugs.

Explanation:

The Scientific Revolution questioned superstition and traditional beliefs that had not been investigated.  The Enlightenment championed this approach to knowledge, emphasizing the use of reason in all matters, in contrast to following superstition or unquestioningly accepting traditional beliefs.  

The Scientific Revolution showed that there are natural laws in place in the physical world and in the universe at large.  Applying similar principles to matters like government and society, Enlightenment thinkers believed that using reason will guide us to the best ways to operate politically so we can create the most beneficial conditions for society.  For many Enlightenment thinkers, this included a conviction that all human beings have certain natural rights which are to be protected and preserved.    This led to changes of approach to how society should be governed -- moving away from ideas like the "divine right of kings" to the establishment of more democratic, constitutional societies.