Supercoiled DNA is a length of DNA that has been subjected to a twisting stress that has distorted its normal shape. As the twisted ladder of the double helix is twisted beyond its normal state, the torsion forces the DNA molecule to deform to relieve the stress.
If the DNA is arranged in a circular shape, as it is among prokaryotes, this torsion causes the molecule to fold one section of its length over another. A similar effect can be observed by twisting a rubber band and letting it twist around itself in response. DNA that has folded in this way is said to be supercoiled.