avyaktaadvyaktayaha sarvaahaa prabhavantyaharaagame |
raatryaagame praleeyante tatraivaavyaktasangyake || 18 ||

With the start of day, the entire manifest (universe) is produced from the unmanifest. With the start of night, it is dissolved into that which is called the unmanifest.
avyaktaat : from the unmanifest
vyaktayaha : manifest
sarvaahaa : entire
prabhavanti : produced
ahaha : day
aagame : start
raatrya : night
agame : start
praleeyante : dissolution
tatra : that
eva : only
avyakta : unmanifest
sangyake : called
Shri Krishna explains the process of cosmic creation in this shloka. When the day of Lord Brahma begins, the entire universe comprising all living and non-living entities wakes up and begins to act. This process continues until the end of his day. Then, the whole universe goes back into the same unmanifest state that it sprung from. Alternatively, the whole universe is born out of Lord Brahma (the unmanifest), and goes back into him.
We can understand this shloka using the analogy of the movie theatre from the beginning of this shloka. The entire animated movie lies unmanifest in the reel of film. When the movie begins, someone loads the film into the projector and turns the projector on. Then, the bright white movie screen comes alive with all the characters in the movie. When the reel of film ends, all the movie characters are “dissolved” from the white screen. This process is repeated next time, and so on and so forth.
Now let us examine the implications. The universe is never really destroyed. It goes into a state of suspended animation, only to be “un-frozen” when the next day of Lord Brahma begins. This is consistent with the law of conservation of energy and matter. Neither energy nor matter is created or destroyed. They just transform from one state to another.
If we know this, and if we also know that every action that we perform always yields a result sooner or later, we realize that the universe works on a set of well-defined laws. Nothing is random, everything is an effect of some earlier cause. Therefore, we can begin to develop the right set of expectations towards the world. Then, while performing actions in a spirit of karma yoga, we will know that only performance of actions is in our hand. The universe, with its set of in-built laws, will take care of the rest. There is no room for worry.
Now, do the living and non-living being have a choice in this process of creation and dissolution? We shall see in the next shloka.