This means Mary's motherhood is divine in nature, not Mary personally. It was a divine gift. It was freely offered, and Mary accepted this divine gift, although she probably thought she was undeserving of it. Otherwise, she wouldn't have been puzzled at Gabriel's announcement. In many places, when Mary is referred to, it says that she pondered the things that were going on and kept them in her heart. Mary cooperated fully with God's will.
I cannot believe that there are so many who want to say that Catholics think that Mary is divine in her nature. She is not God, and Catholics do NOT believe that she is God! You are misunderstanding our beliefs! Perhaps it is difficult to understand, and it is definitely difficult to explain.