The concept of Guna is fundamental in Yoga philosophy. It can be confusing to people with Scientific training. However, it is just a conceptual model. One way of looking at the world. I discuss this issue in detail here. I will try and explain the Gunas in non-technical terms.
The context comes first! Imagine our world 2500 years ago. Telescopes, microscopes, spectroscopes, computers, etc had not appeared yet. All that a Yogi had were his mind and sense organs. His view of the world is constructed from the information coming directly from these organs. His main tool of analysis was introspection (i.e. meditative analysis or Saṁyama in Sanskrit).
This explains how Yoga views the world. Yoga regards the phenomenal world as an ordering of sensory data starting from the gross sensory impressions (e.g. solid, liquid, and heat) to increasingly finer perceptual layers going all the way up to the basis or substrate of the mind. These layers are known as Tattvas.
The mind aggregates and processes the sensory data and creates a picture of the world. In other words, the mind illuminates the world. Though Yoga contends that the world of objects has an existence independent of a my existence, the world I see is a representation in my mind by my mind.
Yogis observed three noticeably different mental states. There are times the mind is sluggish and heavy, overcome by inertia. There are times it is agitated, compelled to do something. Anything! And there are times it is neither sluggish nor agitated and a feeling of calmness, clarity and lightness pervades the mind. The sluggish state is called Tamas, the agitated state Rajas and the state of clarity and lightness Sattva. In my opinion, Sattva is best understood through negation (i.e. neither Tamas nor Rajas). These are called Gunas. Guna in Sanskrit means character or attribute.
Since the mind conceives the world, these three fundamental mental states or Gunas can be used to conceptually describe the entire phenomenal world. Every object we come across leaves an impression in our mind. This impression could leave us in a Tamasic, Rajasic or Sattvic state. Thus the objects are classified accordingly. This is a subjective and context-dependent classification. For example, eating one ripe banana when you are hungry might leave you in a Sattvic state. However, if you eat 10 bananas in one sitting, the 10th banana consumed most likely will leave you in a Tamasic state (i.e. that 10th banana is Tamasic).
When my mind is in Tamasic and Rajasic state, evidently, I am unlikely to perceive and evaluate things clearly and accurately. Therefore, this incorrect perception will more likely lead to actions that will bind me to things in the form of attachment and aversion, resulting in suffering.
On the contrary, the calmness and clarity of Sattvic state pre-supposes and is accompanied by correct knowledge or discernment. When my mind is in Sattvic state, I am likely to perceive and evaluate things correctly and unlikely to commit actions that lead to bondage and suffering.
According to Yoga Sūtras, the canonical text of Yoga Philosophy, the goal of Yoga is to cultivate discernment. Sūtra 2.26 says that unbroken discernment (viveka khyāti in Sanskrit) is the means to liberation. However, being in a state of discernment and being Sattvic are one and the same. One cannot come about without the other. Therefore, one can state equivalently that being in an unbroken Sattvic state of mind is liberation. And this is exactly what Sutra 3.55 states.
The practices of Yoga are meant to bring about this Sattvic state of mind in the practitioner. This helps us to evaluate if a particular practice or a way of doing, for example, an Āsana (posture) is useful for progress in Yoga. Whatever one does must bring about Sattva. Therefore, if you are beat up or if your adrenals are pumped up after your Āsana practice, you might want take another look at your practice.
(Sūtra 2.26) viveka khyātiḥ aviplava hānopāyaḥ.
Unbroken discernment is the means (to liberation).
(Sūtra 3.55) sattva puruṣayoḥ śuddhi sāmye kaivalyam.
A mind established in Sattva resembles the Self and is in a state of liberation.