Nothing happens when I click "improve browser cache"

One of my low performance scores as suggested by Hummingbird was to Improve Browser Cach. The suggestion that follows is that Hummingbird is the quickest and easyiest way to fix this. When I click the improve button, I don't see anything happening. I don't see any improvement results.
I have no clue as to how to do this manually in the Header!
Please explain why the Improve button isn't working, or outline a process for me to do the improvements manually.