2.18 The experienced world consists of the elements and the senses in play. It is of the nature of cognition, activity and rest, and is for the purpose of experience and realization.
In Vedic tradition, there are five key elements: Earth, Fire, Air, Water, and Ether. When those powers combine, you get Captain Planet! Green mullets aside, the Vedas state that all objects in the universe are composed of these elements. In my opinion, elements are not the best translation from Sanskrit. I like to think of them as qualities. Going into more detail would require another couple of posts. I suggest you look up more information on them because they are a big part of Vedic Astrology an Ayurveda
We can look at this from a scientific perspective. The material universe is made up of the physical elements like hydrogen, helium and carbon. The five senses of sight, smell, taste, touch and hearing connect our perception to the environment. At it's most basic level, the human experience is an interplay of these forces.
Or attempts to make sense and put order to the interplay of elements and senses is the process of cognition. When we are not in the process of cognition, our natural state is to be at rest. Yoga works by putting our mind into a state of rest so that we get more control over the process of cognition. In this state of rest, we see the interplay for what it is and can sort it out through meditative techniques.
Sutra 2.18, in a small way gives us a little insight into why we are the way we are. We are here to experience and have realizations from those experiences. Living for the sake of living. Doing for the sake of doing. It's all the same under the sun, as they say.
On an off note, I was thinking of doing a YouTube Channel dedicated strictly to doing Yogic practice. I would give instructional videos on the Kriya techniques I found the most helpful over the last eleven years. I would do this in a blog, but I think videos would be a better format. If you are interested, drop me an email or a comment.