The two strands of a DNA double helix are complementary to each other.
The base on one strand always has the same pairing partner on the other, following base-pairing rules:

• A pairs with T
• G pairs with C

To replicate DNA, the double helix is separated into single strands, each of the parental strands serving as a template to build a complementary strand, using the enzyme DNA polymerase.
At the end of this process, two identical daughter double helixes are formed.