In the first sloka we have seen that Lord Krishna talks about the Tree of the world metaphorically. Its substratum, its characteristics and how this tree is nourished .In the 2nd sloka Lord Krishna further explains the life of the human being in the Tree of the world.
15.2 – Adah ca urdhav prasrtah sthasya sakha gunah pravridah vishaya pravalah
Adah ca moolani anusantatani karmaanu bandhini manushya lokee
Meaning of words:
Adah – downwards
Ca -- and
Urdhav – upwards
Prasrtha – extending/spreading
Sthasya – of the tree
Sakhaa – branches
Gunah ---the 3 gunas
Vishaya Pravalah -- sense objects as buds.
Moolani – root
Anu-santatani -- spreads
Karmaanu bandhini--- product of actions
Manushya lokee – world of human beings
Meaning of sloka
The branches of the tree spread upwards and downwards. The branches are the results of the actions in the world, which are strengthened by the 3 gunas ---raja,sattva and tamo gunas and the tree buds are the sense objects. While the subsidiary roots spread downwards , which is the associated with the actions done in the world. So as the number of actions done increase , the roots spread downward into the world of the human beings. Meaning that the actions are making the world to exist.
So actions are the source of the world, the root of the world. Actions are the cause of all bondages, which create the world, which is sustained by the gunas inherent in a person and the sense objects in the world. Perceiving the world of sense objects with the gunas is root cause of actions. Actions are done in expectation of results , due to effect of the gunas prevalent in us. This inturn leads a person to do more actions .This chain results in actions and fruits of the actions being generated. This creates this world tree. This tree thus has strong roots into the world of the human beings. The substratum of all this roots is brahmam which we have seen in the earlier sloka, realizing which one should cut down this tree of the world, with weapon of knowledge.