The goalmouth is the bit of pitch right in front of the goal, where the goalkeeper lives and where attackers try to poke the ball in. It is not the goal itself, and it is not the net: it is that crowded patch of grass (or ice) just before the line. Commentators love it in a goalmouth scramble — bodies everywhere, the ball bouncing, nobody quite sure who touched it last. You will meet it in football talk far more than in ordinary life. Mouth here is the same picture as the mouth of a river or a cave: an opening you look into, not a part of the face.