each and every diety has been assigned one beejamantra. when the beeja is pronounced correctly and repeatedly – the vibration of the sound invokes the particular power of the diety. These are called “ekakshari mantra” or mantra having one letter or beejakshara.
e.g “hreem” is shakti / maya beeja.
ha- Shiva (the Purusha)
ra- Shakti (the Prakriti)
m (bindu) – removal of sorrow
the meaning of the beeja is “let the united power of purusha and prakriti remove my sorrow”. it seems that because of the basic nature of the beeja – it can be combined with other beejas to create more power to the mantra. Like this the beejas for some other deities are as follows,
KREEM – Mahakali
SHREEM – Mahalakhsmi
HRAUM – Sadashiva
DUM – Durga
KLEEM – Krishna
GAM – Ganapati
HUM – Bhairava (destructive power of Shiva)
and so on. each and every letter of sanskrit is the beeja of a particular deity. so you see garland of 52 human heads (representing 52 letters of Sanskrit) on the body of Mahakali.
But these beejas should not be chanted unless given directly by a guru and properly energised. or those may remain ineffective – if not causing harm.