Sounds exotic, but voicing is just a style of tuning a drum. Whenever we tune a drum, we try to get even tension at every tuning lug. Voicing -- a.k.a. ‘tap tuning’ -- goes a step further by micro-adjusting the tension.
Begin with an evenly tensioned drum tuned to your desired pitch. Then put the drum on a soft surface -- I'm assuming a double-headed drum here. (Your drum stool might be ideal for this, and if you loosen the seat mounting screw, it will serve as a swivel table). Putting the drum on a soft surface eliminates the bottom head from the process and brings out the overtones in the accessible head. For a single-headed drum, put it on a carpet.
Now the interesting bit. Place a fingertip lightly on the centre of the drum head and tap the head about 1 inch from one of the tension rods. You'll get something of a 'ping' sound. Then -- keeping that finger in the middle of the head -- tap the tension rod's neighbour. Is it the same pitch? If so you can move on. Otherwise tension the rods until both spots ring the same pitch. Continue around the drum, back and forth between lugs, until all the lugs are voiced correctly. You can then flip the drum over and repeat on the other head.
The voicing process is not always necessary, but for a temperamental drum it can be just the thing to tame the sound. The technique will work on any drum, and it’s easy to do and easy to replicate.