Librarian employee: I see you've got some books there. Is that the one you were asked to return?
Student: No, I left it in my dorm room. These are books I need to check out today. Is it OK if I bring that one by in couple of days?
Librarian employee: Actually, you need to return it today. That is if you want to check out those books today. That's our policy.
Student: Oh, I didn't know that.
What does "by" here mean?
There seems no idiom like "bring by," so I don't really know how to understand what this preposition "by" here means.