Traditionally speaking Yoga is a means to calm the fluctuations of the mind, when the mind is clear then perception is true, when perception is true then correct understanding follows, correct understanding gives rise to appropriate conduct, and eventually freedom from suffering, which is ultimately based on ignorance.
Samkhya philosophy, upon which Yoga is based, divides the entire universe into 2 parts, Purusha and Prakriti. Prakriti is everything we call reality, thoughts, feelings, ideas, work, yoga, sense of self, weather, space, time, and so on, it is in a state of continuous change. Purusha is that which is able to observe Prakriti, it does not change. Purusha is the part which knows, it is like the blank screen upon which the film of Prakriti is projected
Somehow Purusha has become identified with Prakriti, has lost sight of its own nature, or rather that part of Prakriti which is our sense of self has become so close that we have become confused, forgetting our eternal nature and becoming identified with temporal reality.
Another way is to think of Purusha as Brahman, God, Ultimate Truth, and Prakriti as Creation.
In the Heart of Yoga, TKV Desikachar says that the purpose of Prakriti is to be perceived by Purusha. So the purpose of creation is that ultimate truth can know itself.
Yoga is the path to ultimate knowledge, or knowledge of the ultimate, Purusha perceiving itself through Prakriti.
Everything we can know about anything, is mediated through consciousness, reality is an event in consciousness, we can say that Reality is consciousness dependent. What ever theory of evolution, cosmogenesis, materialism, existentialism or whatever religion any person believes to be true, is true because he or she is able to be conscious of it. Consciousness precedes reality.
Even physics, the most objective of sciences, has revealed that solid matter is 99.999999% empty space, and that little bit which is left is mysterious and elusive!
So when Yoga says it is the cessation of mental activity, or the fluctuations of mind, it is actually saying that Truth is perceived when the mind becomes clear like a calm lake and reflects the light of consciousness, which is the reality upon which all other definitions of reality are based.
So this is Yoga, the Realization of the primacy of consciousness, about how things actually are, how it could not be different, it is not about changing what we are, about arriving at a new state of being, but the simple realization of the actual present state.
Well, for one thing, when we understand this, understand our essential nature as being beyond space and time, never born and never dying (birth and death are also events in consciousness), then we are able to act and be differently in the world, at the very least we have a counter point to the trials and tribulations and suffering that comprise a large part of what we call life, we have a refuge, a place in paradise where our true self is always abiding. We feel safe, without fear, and that translates into being able to enjoy life more.