In beginner level, people think that violin sound depends mostly on the left hand and on correct notes and positions. But really violin sound depends on the contact between the string and bow.
The bow behavior is inconsistent at first. It reacts too much on little changes and it feels unpredictable. Even the slightest change in bow position makes significant difference in the sound. That is the big first surprise for many beginners, as the expectation is that sound depends mainly on correct notes, but the reality is that sound depends mainly on precise and physical movements.
Basically, bow is the tool for controlling the sound. When too much pressure is applied, the sound gets ugly. When too little pressure is applied, the sound goes weak and may even disappear. When bow speed is irregular, the sound becomes shaky. All these parameters need to be adjusted together and it may take a long time before you master a consistent, clear sound.
Also there is a concept called contact point. Violin doesn’t have same sound when you bow at every place between bridge and fingerboard. Moving toward bridge produces more forceful sound and moving toward fingerboard creates softer sound. Often beginners don’t realize they are changing contact point without knowing about it and so, it creates uneven tone.
The bow is really hard to control because not everything is purely physical. When you try to make steady tone, you try to feel the resistance of the string and hear what is the resulting sound and then adjust it in real time. So that is a loop between physical movement and feedback and it takes a while to stabilize. Sometimes the feedback may come to fast or too small so brain finds it difficult to understand the sound and connect it to what is happening in the body.
Another misunderstanding is to think that applying more pressure helps to improve sound. But sound is mainly about balance rather than strength. If you push too hard then string can’t vibrate properly, or if you push too little then the string can’t move at all. So what you actually want to achieve is proper weight transfer with consistent movement instead of pushing as much as you can. That balance between bow pressure and weight is not an idea that you can just learn through the explanation but you have to learn through physical repetition.
Eventually a strange thing starts happening. Your hand gets used to the level of pressure that it takes to create a certain sound and starts to learn to notice the level and speed of sound. The more you get used to it the more your brain starts to associate the sound to the feeling. And once this happens, your hand starts to predict the sound before you make it. So it becomes easier to get better sound.
There is also a point when sound becomes stable and stops feeling like random. If you do a certain movement you may get different result. But once you start to understand the movement, you may begin to notice that same movement produces same result and you start to hear a relationship between cause and effect, between the sound and how you are playing. Then you start to understand that you do not just produce sound, you are actively shaping it.
But it is also important to realize that great sound is not about the complexity or speed of the note but about the control in a simple way. For example you can make even a simple open string very beautiful or expressive when you bow with good control. So that is why most violin teachers emphasize on long tone. It lets you concentrate only on making clear and pure sound, because with long tones you have no other issues to deal with.
Ultimately sound on violin is not one skill but a combination of many small skills such as pressure and speed and angle and contact point and listening. Once they begin to fall into place then the sound stops feeling out of control and begins to feel responsive. And at that point your violin starts feeling like an instrument that you control rather than one you fight with.