“Contingent take back-up” means you are “Under Contract” but there are “contingencies” such as final loan approval and/or inspections which could affect the closing.
Buyer and seller have to come to a written agreement following the inspection (Repair, Replace, Treatment-RRT), within a certain number of days or the contract is void. It is best to keep the listing “live” and available for showings until all contingencies have been met.