A speech sound where the airstream is from the lungs is constricted or completely blocked.
For example, completely blocked (e.g. /t/, /p/), partially blocked (lateral, e.g. /l/), or where the opening is so narrow that the air escapes with audible friction (fricative, e.g. /f/), or the airstream is blocked in the mouth but allowed to escape through the nose (nasal, e.g. /m/)
Consonants can be either voiced or unvoiced, e.g. /b/ is a voiced bilabial plosive, while /p/ is an unvoiced bilabial plosice.