"DNA" is a kind of complex molecule inside of cells which controlls the cell.

DNA is made of a long chain of atoms which are connected to each other. The word "DNA" is uncountable, but because of the long, thin shape, you can count it by talking about individual "strands":

How long is a strand of DNA?

The word "strand" is also used to count other long, thin things:

a strand of hair

a strand of pearls

a strand of spaghetti