Purport

This Shloka is again, one of the most important Shlokas of Shrimad Bhagvad Gita and answer one of the most asked questions,that is “why can’t we see God”?

In Shloka just before this Shloka, Bhagvan Shri Krishna described two important things
1. All living and non-living entities in this material world are subject to three Gunas, i.e Satva, Rajasa and Tamasa.
2. Though all three Gunas originated from Shri Krishna He is beyond the periphery of three Gunas.

In this Shloka Bhagvan describe the secret that every living being since are deluded by gunas (modes of nature) and Shri Krishna, the God is beyond Gunas and hence human are not able to witness, realize or see God.
To understand this in further details we should try to understand our body and actions. Generally, we think we see something, we hear something, we eat, we sleep, etc. But go into further details. For example see: What happens? Some light rays fall on our eyes, eyes send some signals to minds and mind interpret those signals which we call as seeing. The same happens with hearing and other sense of activity. This means what we understand is actually the interpretation of the mind. We don’t even know what we see, hear, etc even exist in reality or not. But we assume that because everything is described from that perspective.

At the second level, try to understand the mind. What is this? Isn’t it just the combination of some biological cells which formed a bigger organ-like mind and do the complex operation of memorizing, understanding, etc. The mind simply is a physical entity and so his the whole body.
The summary is that this whole body including is nothing but a complex combination of physical material. In all our senses we simply comprehend the complex phenomena of the material change. This what this shloka describes as “The entire universe is deluded by these three conditions originating from the Gunas”

We humans can understand only the material world and its phenomena, But God is beyond this material phenomena. Hence we cannot witness God using this material senses.

A human can witness God with their eyes only in one situation and that is when God himself acquires a material form and come among us in way we can see him. This process is called Incarnation. Many people who may want to understand the significance of Incarnation can get the answer from this.

Another way to witness God is for a human to acquire a state where he detaches himself from this material world. That state can be acquired by the rigorous process of Yoga. The same has been described in detail in Chapter 6. There is yet another process, and that is the Bhakti, in that case, God appears to his devotee in the form devotee wishes to see.