You can view the profiles you have Skipped, Liked or sent an Intro to in the 'People you have seen' screen.
You can access this tab by tapping the reverse clock icon at the bottom of the screen. If you wish to Like a person you have previously skipped or take another action just click on their profile and away you go!
After a while these profiles will be blurred out unless you have a Premium subscription where they will remain visible.