Beeja mantra is the seed mantra in Hindu religion. The best known Bija mantra is Om and it is called Maha Beeja, the great seed mantra. It is called so as it is the source of all other Beeja mantras.
The beeja mantras, other than om, are particular sounds caused by the letters of the Sanskrit alphabet. These evolve out of the general sound which underlies all particular sounds.
Pranava (om) and the beeja mantras as pronounced by the mouth are the articulate equivalent of the primal dhwani (sound). The sound equivalent of the Brahman in Pranava mantra and the various forms with attributes of gods and goddesses (devas and devis) are the beeja mantras.
A beeja mantra is a mantra of single letter such as kang, which is composed of the letter k (ka) together with the consonant Chandra bindu (the sound ‘m’ in the form of nasal breathing) which terminates all mantras. The ‘m’ which ends the bija is sounded nasally, high up in the bridge of the nose, and never reaches the lips.
Though, strictly, the beeja is of one letter, the seed from which the mantra springs, some short and etymological vocables such as ‘hring’, and ‘phat’ are also called beejas.
Beeja mantras have no meaning according to the ordinary use of language. The initiated know that their meaning is the own form of the particular gods and goddesses, whose mantras they are. They are a form of the subtle power of the creative dhwani, which makes all letters sound and which exists in all that we say or hear. Every letter, syllable and mantra is a form of the Brahman. All letters are forms of Shakti.