A topoisomerase is one of a group of enzymes that unwind and wind DNA in order to help the creation of DNA structures.
DNA is in a helix/spiral which helps make the structure of DNA strong and stable. However, when undergoing DNA replication, the spiral needs to be untwisted, and when spirals are being untwisted knots, and breaks can happen. When studying the structure of DNA, James D. Watson and Francis Crick knew that there was need for an enzyme to overcome this problem.
Topoisomerase binds to either side of the DNA strands and allows the DNA to be untangled or unwound.