As creation developed, it formed three underlying principles that uphold all of life. These are the law of creation, maintenance and dissolution. Everything in life is born or created, it lives and then it dies.
Ancient sages observed that the tendency to act, rest and balance is within all living entities. Sattwa, rajas and tamas (know as the Gunas) are the three modes of the universe inherent in every particle, without which we can't exist. Generally these three modes are applied to understanding the mind.
Sattwa - balance and harmony. Emotional stability and detachment. Satwic people tend to possess an effortless wisdom reflected in their actions.
Rajas - accounts for motion, energy and activity. Rajas is essential to get things done.
Tamas - accounts for intertia. Tamas is the force behind the power to rest, relax and sleep. It provides grounding and stability.
Life is a play of the gunas. Sometimes due to eating heavy food, we may feel lethargic and tired (tamasic). A child excited about their birthday who may not be able to sleep will have excess energy (rajasic). A person after practicing meditation and morning prayers may feel peaceful and ease (sattwic).
By understanding the nature of the gunas and respecting the ebb and flow of the gunas, we can live our life in balance and harmony.