Definition:
Complementary distribution is when two phonemes in a language do not appear in the same phonological environments. When two phonemes are in complementary distribution, they are likely allophones of the same phoneme. If two phonemes do appear in the same phonological environment, they are not in complementary distribution and are likely two different phonemes (i.e., contrasting phonemes) (Zsiga, 2013, p. 205)
If two sounds are in complementary distribution, they are said to be predictable and non-contrastive (Zsiga, 2013, p. 205).
Examples:
INLP Linguistic Glossary 