1. Open and closed refer to the position of your tongue when pronouncing vowels. If your tongue us high (near the roof of your mouth), then the vowel is said to be closed because your tongue fills the cavity of your mouth. If your tongue is low (near your bottom teeth), then the vowel is said to be open because the cavity of your moth is open. See the chart here for open and closed vowels:
Vowel - Wikipedia, the free encyclopedia
3. Bi- means 2 and labial means lips. So bilabial means 2 lips. Sounds that are produced using both lips are [m], [p], and [b]. Only two of those are plositives, sounds that require you to build up air at the glottis (located in your throat), and then release it quickly. LIKE AN EXPLOSION!
The sound [m] doesn't do this, so it's not a plositive.
