a sheep: the flock animal raised for wool, milk, meat, and sacrifice, and by extension people pictured as sheep in a shepherd's care
The word simply means a sheep, the everyday flock animal of the ancient world. Sheep were among the most valuable things a family owned: their wool made clothing, their milk was food, and they were the standard animal brought to the altar. So when the Gospels mention buying and selling sheep in the temple courts, or rescuing one that has fallen into a pit on a rest day, they picture a real and prized animal.
From that everyday picture the New Testament builds its most common image for people. A flock of sheep cannot survive on its own: it drifts, wanders off, gets lost, and is easy prey with no one to guide and guard it. So the writers call leaderless crowds sheep without a shepherd, and describe God's people as a flock that has strayed and then been gathered back. The word never stops meaning sheep; it simply points at people through the animal.
This image reaches its height in Jesus as the good shepherd who knows his sheep by name and lays down his life for them, and in his charge to Peter to feed his sheep. The same picture runs back through the Hebrew Scriptures, where God is the shepherd of Israel. Reading these passages well means holding both layers together: the real animal and the people it stands for.