The end of a DNA strand that terminates with a free hydroxyl group is the 3′ end.
This name comes from the third carbon of DNA’s deoxyribose sugar. At the 3′ end, the sugar’s 3′ carbon carries an exposed hydroxyl group, or 3′-OH. The opposite 5′ end normally carries a phosphate attached to the sugar’s fifth carbon. These chemically different ends give every DNA strand directionality.
The two strands in the DNA double helix run antiparallel: one runs 5′ to 3′, while its partner runs 3′ to 5′. DNA polymerase can extend a strand only by adding a nucleotide to an available 3′-OH, so new DNA is synthesized in the 5′-to-3′ direction. This is also why a primer must provide a free 3′ hydroxyl group.
A related source of confusion is the 2′ position. DNA has a hydrogen there rather than the hydroxyl found in RNA’s ribose. The 3′-OH is also the key chemical group used in Sanger sequencing reactions.