Yoga's whole aim is chitta vritti nirodhah, the stilling of the fluctuations of the mind. Two terms make that practical.
The mind-field (Chitta) is the whole field of mind-stuff: consciousness, memory, impressions, and everything that arises within them. Not just "thoughts," but the entire mental substrate, the surface on which experience appears.
A fluctuation (Vritti) is any movement of that field. The tradition counts five kinds: right knowledge (pramana), misconception (viparyaya), imagination (vikalpa), sleep (nidra), and memory (smriti). The problem is never the vritti itself, since the mind will always move. It is identification with the movement, mistaking the ripple for yourself, that practice slowly loosens.
Picture a lake: disturbed, its surface distorts what it reflects; still, it shows things as they are. The whole teaching opens the Yoga Philosophy page, and is shown as stilled water on the Introduction.