chicken's beak; mouth of a chicken
Literal meaning: the beak or mouth of a chicken. Rarely used alone; mainly appears in the set phrase 鶏口牛後.
leader of a small group; big fish in a small pond
Figurative use, almost always in the context of the proverb 鶏口牛後 (better to be the head of a small group than the tail of a large one). Not used as a standalone word in modern Japanese.
See also: 鶏口牛後 (けいこうぎゅうご) · 牛後
In the proverb 鶏口牛後, 鶏口 means the leader of a small group.
He quit the large corporation and started a small company, choosing to be a big fish in a small pond.