The 5′ and 3′ mean "five prime" and "three prime", which in turn indicate the carbon numbers in the DNA’s sugar backbone. The 5′ carbon has a phosphate group attached to it and the 3′ carbon a hydroxyl group. This asymmetry gives a DNA strand a "direction". For example, DNA polymerase works in a 5′ -> 3′ direction, that is, it adds nucleotides to the 3′ end of the molecule (the -OH group), thus advancing to that direction.