While the common (or polished interpretation that was presented to non-initiates who would otherwise be scandalized lol) meanings are already listed, I was wondering whether or not to speak about the inner meaning related to sexual alchemy but could not resist commenting haha
Mani - Thunderbolt Jewel or Vajra
Padma - The Flower or Vulva
Hum - Penetration
And what drives this activity? The driver as well as the destination → OM
Tantra developed in a highly orthodox environment and faced a lot of opposition thanks to some “radical” ideas that were considered bizarre by orthodoxy. So, secret underground societies were formed that refined and practiced these techniques (Guhyasamaja). They also developed a technique or language called “Sandhyabhasha” - the twilight language. Mantras, and techniques were described in highly metaphorical, coded language and only the initiates knew how to derive the actual mantra/technique from manuals. Following the same paradigm, Padma (lotus) in Tantra means the Female Organ, and Vajra or Jewel (Thunderbolt) means virile Male Organ. The coming together of the two in a specific way was the core technique of sexual alchemy, an integral part of Tantra. These techniques were also coded because such things involving sex, intoxication, etc., are easy to abuse and pervert.
This mantra originally was intended for sexual alchemy in highly-advanced rituals where the breath is stilled, the mind is focused on the mantra with specific visualization and technique. The result is not a localized orgasm driven by ejaculation, but a Crown-gasm accompanied by secretion of nectar (these are experienced for real, and not imaginary, I can vouch by personal experience).
In fact, this is just the first of a series of mantras 
The next one is oṃ manidhāriṇi vajriṇi svāhā (OM Oblations to the one who has encompassed/assimilated the essence of the Thunderbolt) - this is the next mantra recited when the Crown-gasm arises, where the Female Yogini focus on the bliss of the Crown-gasm, while also remaining aware of this mantra being recited in conjunction with the regulated breath. The Male Yogin has his mantra to recite similarly.
Normal orgasm tires people and puts one to sleep. This Crown-gasm has the opposite effect. It wakes us from Cosmic Sleep - we wake up from sleep, from the dream of the self as well as that of the Cosmic Mind, and then reality becomes starkly evident. When the sleep, the dream, etc., collapse into a singularity, one begins to see the real nature of things - that the observer and the observed are not different, everything is one. This Awakening is contrasted by masters of the yore with the sleepy effect of the mundane orgasm.
Note: Focus on breath and let go when listening to this field, you will slowly notice bliss arising 