Text scaling is a tool that controls the size of text on different breakpoints, so that your text always looks good whether viewing from your desktop or phone. For example, when you design how your header looks on desktop, the text may be too big when you switch and look at it on mobile.
With text scaling, you can choose the minimum and maximum size of the text, that way the max size will be shown on desktop and the min size will be shown on mobile. Once you set your text to scale, you can see how it changes by switching between breakpoints.
Note: Changing the value of the text sizes from a lower breakpoint (e.g. tablet or mobile) breaks the text scaling across breakpoints.