Om is the Universal Sound or the
beginning of all sounds (mantras).
Mani means the most precious jewel (ie Buddha nature).
Padme means lotus which symbolizes purification of one mind.
Hum means exclamation in wonder
This mantra then, means
"Here comes the holiest of all sounds.
You have the most precious sound inside of you, the Buddha nature,
which can be realized through ultimate perfection of the mind.