Despite what could be implied from what appears (at the time of writing) in the heading for this attraction, you don't need a guided tour to visit this site. We actually bought the combination ticket that includes also the cathedral dome climb, Giotto's Tower, the Baptistry and Cathedral Museum.
This crypt-excavation site is the remains of the Cathedral of Santa Reparata, the precursor of the current Duomo, as well as earlier Roman remains. It's a surprisingly large site all under the present cathedral, and is reasonably-well explained. You can also see the tomb of Filippo Brunelleschi, the architect of that huge dome that sits way above you, unseen from the crypt.