Although the doctors had been able to 'keep her illness at bay' [=to prevent her illness from becoming worse] for a few months, the disease soon began to spread again.
Armed with a gun, he 'held the police at bay' [=he did not allow the police to arrest him] for 10 hours.
The soldiers 'kept the attackers at bay'. [=they did not allow the attackers to come closer]
(..) A gift to the foes of Mordor. Why not use this Ring? Long has my father,the Steward of Gondor... ...kept the forces of Mordor at bay. By the blood of our people... ...are your lands kept safe. Give Gondor the weapon of the enemy. Let us use it against him. You cannot wield it. None of us can. (..)2001 The Lord of the Rings: The Fellowship of the Ring