A bilabial sound is a consonant you make with both lips. Say /p/, /b/, or /m/ slowly and you can feel it: the two lips do the work. /p/ is a lip pop with no voice; /b/ is the same shape with voice; /m/ keeps the lips closed and sends the sound through the nose. English also has /w/, which starts with both lips, though teachers sometimes argue about whether to file it here. This is a classroom word for how we make sounds, not a word for the street. You will see it in pronunciation books. Though actually, learners often think /f/ and /v/ are lip sounds too; those use the bottom lip and the top teeth, so they belong in a different group.