The History page is, as its name suggests, the place where you can view a list of all the profiles you have seen, along with the decision you made on each one.
To get here, select Members from the top menu, then History from the submenu.
If you previously Removed a profile, or Saved someone by marking them as Maybe Later, you are able to change your decision by opening their profile and choosing a different option.
If you previously sent someone a Message, you can't change your decision, as the other member will already have been sent this message.
You will also notice filters on this page, as shown below. They include All, Messaged, Saved, Skipped and Dismissed.
Select a filter to bring up the list of people within that category. For example, if you would like to see a list of all the people you have messaged, select the Messaged button. If you would like to see who you have saved, you can do so by selecting the Saved button.