Bg 15.2

adhaś cordhvaṁ prasṛtās tasya śākhā
guṇa-pravṛddhā viṣaya-pravālāḥ
adhaś ca mūlāny anusantatāni
karmānubandhīni manuṣya-loke

Synonyms: 

adhaḥ — downward; ca — and; ūrdhvam — upward; prasṛtāḥ — extended; tasya — its; śākhāḥ — branches; guṇa— by the modes of material nature; pravṛddhāḥ — developed; viṣaya — sense objects; pravālāḥ — twigs; adhaḥ— downward; ca — and; mūlāni — roots; anusantatāni — extended; karma — to work; anubandhīni — bound; manuṣya–loke — in the world of human society.

Translation: 

The branches of this tree extend downward and upward, nourished by the three modes of material nature. The twigs are the objects of the senses. This tree also has roots going down, and these are bound to the fruitive actions of human society.

adhaś cordhvaṁ prasṛtās tasya śākhā

Krishna is saying things are not simple here, some roots are going down and some upward.

adhaś cordhvaṁ prasṛtās tasya śākhā

The branches go upward as well as downward, this represents how a living entity can move up and down in the material existence. The creation has begun from Brahma so the roots are up and from there, things move downwards. Brahma ji is the first created living being, and living beings fall and then stay in material existence and they go in various species and various directions. With their transmigrated trajectory they can come downward or can go upward. The branches here represent how the tree expands.

guṇa-pravṛddhā viṣaya-pravālāḥ

guṇa-pravṛddhā, they are nourished by the modes so this is the connection. Every tree needs water for nourishment, when we choose a particular desire accordingly to the three modes then accordingly our material existence expands. Accordingly, we move forward in life.

viṣaya-pravālāḥ, viṣaya- sense objects; pravālāḥ- twigs so viṣaya-pravālāḥ refers to sense objects as the twigs. 

adhaś ca mūlāny anusantatāni, the roots also go downward 

The whole tree is very complicated. When we use the word ‘root’ we use it in many senses, one meaning ‘from where things begin’. Where we come from. The roots of all our actions are desires; the thought is the seed of the deed. What we have to do is, we have to change our thoughts if we want to change our actions. Also conversely if we do certain actions it affects our thoughts. So based on actions, thoughts are changed which means roots are changed. So we go by our Karma upwards or downwards and our conceptions change based on how we act. Sometimes we act positively and sometimes negatively so accordingly we go up and down. 

adhaś ca mūlāny anusantatāni
karmānubandhīni manuṣya-loke

karma + anubandhīni- a person becomes bound by Karma. manuṣya-loke- in human existence. The Law of Karma works in different ways on different levels of planets. The earthly planet is basically for doing Karma and the heavens and hells are for enjoying the fruits. Even in heaven if they offend devotees they have to suffer. But there is much more freedom in heaven than on the earthly planet, the Laws of Karma are different there.  

karmānubandhīni manuṣya-loke

So bondage is created in manushya loka. So in this ways living entities are bound in human society. 

SP Purport: In the lower parts, there are variegated manifestations of living entities – human beings, animals, horses, cows, dogs, cats, etc. This planet of human beings is considered the field of activities.