Generally, two elements a and b are in complementary distribution if a, but not b, occurs in those environments where on general grounds we may expect both a and b, while b, but not a occurs in the complementary set of environments.
In phonology, complementary distribution is often taken as an indication that two superficially different elements are one and the same at a deeper level. Two sounds /a/ and /b/ are in complementary distribution when one of the two (/a/) occurs in all environments except those in which /b/ occurs and vice versa.
In English [p] and [ph] are in complementary distribution, since [ph] occurs syllable-initially when it is directly followed by a stressed vowel (cf. pin [phin]), whereas in all other positions [p] is found.
In Hindi, however, [p] and [ph] can occur in the same position and are distinctive.