Two pickets were stationed at the entrance to the camp.

pickets 是名词 picket 的复数形式。picket 原指"尖木桩"，引申为"哨兵"或"警戒线"，因为过去常用尖木桩作为防御工事或标记警戒区域。在军事语境中，picket 特指"哨兵"或"前哨"。