Verified answer

The numbers refer to the numbering of the carbons on the deoxyribose ring.

The 5' carbon has a phosphate group bonded to it.

The 3' carbon has a hydroxyl group bonded to it.

When you see a single stranded DNA sequence written out, the convention is that it's written 5' first (5' is the left end).

During transcription, DNA is read from 3' to 5'. The complementary RNA is read from 5' to 3'. If you remember, transcription occurs in the nucleus, and that is when the DNA double helix is unwound by RNA polymerase, which creates the complementary RNA strand. The same enzyme puts the DNA double helix back together once RNA is formed.