Firstly, what is contrast? It is the difference between two things. If you increase contrast, you are increasing that difference. When you have a line in your photo, either side of the edge of that line is brightened and darkened to increase the perception of sharpness.
Too much contrast can also have a negative effect. Your photo can become grungy, skin tones can look unnatural and existing artefacts in your photo more evident. Paying attention to the adjustment results will help to determine which tool to use and how much you can apply.
How to use the Tonal Contrast tool
Tap on the Edit pencil icon, then Tonal Contrast. When you open the Tonal Contrast, you will notice an instant improvement in your image. A default adjustment is applied immediately. You can adjust these to your personal preference and even reduce the values back to zero.
A couple of quick navigation tips: Tap and hold the screen or the before and after icon to compare the edit. Tap on Adjust to reveal the submenu or swipe up and down on the screen.
High, mid and low tones – are specific tonal ranges of the image that you can increase the contrast (bright, dark and areas in between). Editing the mid and low tones are where you will notice the strongest effect.
Related article: Snapseed Curves tool enhances colour and tones - makes your photos pop