Usually, when rats come out during the day, it is a sign that there is a large infestation of rats and that food supplies are short.
This is because rats are nocturnal animals and they will usually only be active during the night so that they can use the cover of darkness as a method of defense.
If you see a during the daytime, it means that he is forced to search for food and water during times that are outside of his comfort zone, because he could not find these items sufficiently during the night.