Anti-parallel is a term associated with the formation of the DNA double helix and the secondary structure of proteins, from amino acids. It suggests, in both cases, that the two strands of molecules are the inverse of each other and complementary.
Firstly, the DNA double-hleix is comprised of several components; a sugar-phosphate backbone (comprised of a pentose sugar and a phosphate group), and nucleotides. These nucleotides bind via hydrogen bonds and are complementary to ech other (cytosine withguanine and adenine with thymine). The double helix contains genetic information in the form of DNA, which is stored in the nucleus of eukaryotes. The reason for DNA to be in an anti-parallel formation is due to it's replication in a 5' to 3' direction. This is because the leading and lagging strands are synthesised in opposite directions; DNA polymerase moves in a 3' to 5' direction.
Also, beta sheets in the secondary structure of proteins are formed in an anti-parallel structure. These are made of multiple peptide chains and bound together with hydrogen bonds. This anti-parallel structure is more optimal for the protein as the hydrogen bonds are more regular and therfore stronger. These hydrogen bonds are between the peptide strands rather than within them.