"Second Looks" aka "Revisit" are formal days and/or weekends (depending on the school) where all accepted applicants are invited back to get to see the school again (and meet and get a vibe for future classmates). They are hosted by the school and will involve a variety of activities for you to get to know the school (curriculum presentations, financial aid presentations, meeting the deans, etc..) as well as for you to get to know your classmates (ice cream socials, bar hopping, clubbing, etc.. these are usually planned by current 1st and 2nd years). Some schools will pay for you to fly out for second look/revisit (Vanderbilt comes to mind), others won't (like Columbia and NYU). And some schools do not have second looks/revisit at all (like Tufts).
Good luck OP!