Bhagavad Gita Verse 69, Chapter 2

yaa nishaa sarvabhootaanaam tasyaam jaagarti samyami |
yasyaam jaagrati bhootaani saa nishaa pashyato muneh || 69 ||
 
That which is night for all beings, the balanced individual is awake in that. And that in which all beings are awake, the person of contemplation views as night.
 
yaa : that
nishaa : night
sarvabhootaanaam : for all beings
tasyaam : in that
jaagarti : is awake
samyami : balanced individual
yasyaam : and that
jaagrati : are awake
bhootaani : all beings
saa : that
nishaa : night
pashyato : views
muneh : person of contemplation
 
Here we encounter one of the most poetic shlokas in the second chapter, again, having several layers of meaning. Let us try to understand it to the best of our ability. First, let's consider an example that will make the understanding of this shloka easier.
 
When a lay person and a professional artist enter a museum, both of them find joy in appreciating the works of art. The lay person may get excited about seeing how accurately an artist has painted a portrait. But the professional artist may find joy in more subtler aspects of the very same painting, e.g. what brush strokes were used, which time period was the painting commissioned in, what were the influences and so on - aspects that the lay person is probably totally oblivious to. 
 
So therefore, in this shloka, Shri Krishna is speaking about two groups of individuals: ones who maintain the state of equanimity and ones who don't. Both groups have to live in this world of material objects, and both of them have to face ups and downs in life. The key difference in both groups is their vision.
 
For most individuals, the world of material objects is their end goal, they are "awake" to it, and the eternal essence is like night to them. But for the individuals of equanimity, the world of material objects loses importance - that world is like night to them. They are awake to the timeless, changeless eternal essence.