Chitta is the mind-stuff, and Vrttis are the waves and ripples rising in it when external causes impinge on it. 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 Chitta, and the waves are the Vrttis.
A great mantra to repeat during meditation is, “Yogash chitta vritti nirodha.” This translates to "Yoga is the silencing of the modifications of the mind.”
This post uses material from the following Wikipedia article: https://en.m.wikipedia.org/wiki/Vritti. This article is released under the Creative Commons Attribution-Share-Alike License 3.0.