The seven summits are the highest mountains on each of the seven continents. Rob and Jo summited all of them together and at the same time and on their first attempt and took 1 year and 38 days to complete them.

Re the 8th summit:

Rob & Jo also climbed Carstensz Pyramid, which is considered by some to be one of the Seven Summits.  The difference being that while Kosciuszko is on the continent of Australia, Carstensz Pyramid is on the  continental plate of Australasia.