In the very early morning, my husband and I heard a lot of noise at the front door and I went to see if Kate was locked out.

locked 是动词 lock 的过去分词形式，lock 的意思是"锁上、锁住"。这里用过去分词表示被动状态。 out 是副词，表示"在外面"。 locked out 是一个固定短语，意思是"被锁在外面"，指因为门被锁上而无法进入的情况。