So you understand what is meant by citta. It is the mind-stuff, and vrttis are the waves and ripples rising in it when external causes impinge on it. These vrttis are our whole universe. The bottom of the lake we cannot see, because its surface is covered with ripples. It is only possible when the ripples have subsided, and the water is calm, for us to catch a glimpse of the bottom. If the water is muddy, the bottom will not be seen; if the water is agitated all the time, the bottom will not be seen. If the water is clear, and there are no waves, we shall see the bottom. That bottom of the lake is our own true Self, the lake is the citta, and the waves are the vrttis. Then, at last, when the waves cease, and the water of the lake becomes clear, there is the state called sattva, serenity, and calmness. This citta is always trying to get back to its natural pure state, but the organs draw it out. To restrain it, and to check this outward tendency, and to start it on the return journey to that essence of intelligence is the first step in Yoga, because only in this way can the citta get into its proper course.
Sri Swami Vivekananda