小蜜不知道rolling？

发了个邮件给小蜜
Does your department have a "rolling admission" policy?
have a "rolling admission" policy是我在其他学校看到的，所以以为美国人都知道这些
可是她回复
Not sure what you mean. If you are asking if we don't admit you in one
Semester can you just be considered for the next, no our department does not do this

我的英语不好，这个rolling应该怎么解释？