The meaning of this line is that it is supposed to be light. It is daytime and therefore it is supposed to be light.
In this scene, all kinds of crazy and unnatural things are happening. Ross and the old man think that these things are happening because Duncan has been murdered. The idea is that nature itself is upset because of the killing of the king because that upsets the natural order of things.
If you look at the lines before the line you cite, you can see that this makes sense. The lines are:
By the clock ’tis day,
And yet dark night strangles the travelling lamp.
Is't night's predominance, or the day's shame,
That darkness does the face of earth entomb,(10)
When living light should kiss it?
Ross is pointing out that it is supposed to be daytime but the face of the earth is in darkness when living light is supposed to kiss it (when it is supposed to be light).