What does complementary base pairing mean?

A DNA molecule has 2 strands. Complementary base pairing is when a base on one strand 'matches/is complementary to/ fits with' the base on the opposite strand.

An easy way of remembering which bases are complementary is the word 'taxi' but instead 'TAGC'

T and A are complementary and have 2 hydrogen bonds between them

G and C are complementary and have 3 hydrogen bonds between them.

Hope this helps!