DNA strands are complementary, meaning that the strands "match up." Certain bases can only pair with each other. A only pairs with T, and vice versa. C only pairs with G, and vice versa.
For example, say one sequence is A-T-T-C-G-A
The complementary strand would have to be T-A-A-G-C-T because A-T and C-G always go together.