A copy of the DNA is made just prior to when a cell is going to split to create two cells. In order for this replication of DNA to take place, the DNA has to be in an orientation that will allow the replication machinery to make a copy. Our DNA is double-stranded, with the strands being held together by hydrogen bonds. The normal structure of our DNA when it is not being copied is a double helix. This looks very similar to a winding staircase.
In this normal form, the DNA cannot be copied. DNA helicase is needed in order to open the DNA to expose the nucleotide bases that are used as the template for replicating the DNA. The area of the DNA that is opened by DNA helicase is known as the replication fork because it looks very similar to a fork in the road.