Anti-parallel DNA strands are the single strands that sit at the end of each DNA molecule. One end is called the 3 prime and the other is called the 5 prime.
Anti-parallel DNA strands that rest at each end of the DNA helix run in opposite directions, hence their name. The 3 prime and 5 prime names come from the notation for the sugar carbon atoms that keep the phosphodiester bonds together. A DNA strand is anti-parallel if it runs parallel to another strand, but turns in the opposite direction. The head of one strand runs parallel to the tail of the other.