He went down on his knees in front of the altar.

knees 是名词 knee 的复数形式，knee 指"膝盖"，是连接大腿和小腿的关节部位。 went down on his knees 是一个固定短语，字面意思是"膝盖向下"，实际表示"跪下"的动作，常用于表示尊敬、祈祷或恳求的场合。