On a violin, there are two types of tuners. There are the large black tuning pegs that anyone can easily see, and there are also fine tuners. These are very tiny and are located on the ends of the string that are closer to the chin of the player. The job of both sets of tuners is to adjust the tension in the string in order to produce a specific note. On a violin, these notes are G, D, A, and E. When the string is tighter, it produces a higher pitched sound. when it's looser, the sound is lower. Most often when tuning, strings need to be tightened a little because colder temperatures cause the wood in a violin to contract, leading the strings to loosen slightly.
More often than not, the small, fine tuners are what is being used to tune the instrument. This is because since they are so small, they only can tighten or loosen a string a little bit, making it easier to tune to an exact pitch. The large tuners are only used when the strings are so out of tune that the fine tuners won't do anything. These are much harder to use because it often gets worse before it gets better. These tuners are only held in place by the string wrapped around it and the wood it is inside of. They will stay in place if you don't move them, but there is a lot of tension in the string. If you try to tighten a string with one of these tuners, sometimes it will end up falling even more flat because there is not enough friction to oppose the motion of the tuner rotating as the string tries to loosen. This is why I have a tendency to ask Ms. Murrell to help me tune if it's that bad.