I think this means that nucleotide sequences at the restriction sites are the same when read forward or backward. Restriction sites, or restriction recognition sites, are locations on a DNA molecule containing specific (4-8 base pairs in length) sequences of nucleotides, which are recognized by restriction enzymes. Palindromic sequence is a nucleic acid sequence on double-stranded DNA or RNA wherein reading 5' to 3' forward on one strand matches the sequence reading 5' to 3' on the complementary strand with which it forms a double strand.