During the fire drill, make sure not to lock anyone in the building.

lock 是动词，意思是"锁上、锁住"。 in 在这里是介词，表示"在...里面"。 lock in 是一个短语动词，意思是"把...锁在里面"，强调将某人或某物关在某个封闭空间内无法出来。