Contrast is something that is not often considered when designing a website, but it can make your site fail accessibility checks.
Contrast, in terms of accessibility, is when you have one coloured element sitting on top of another.
For example, this might be black text on a white background or light grey text on top of a darker grey background (think buttons on your website).
If the contrast between the two colours is not sufficient, it can make the text elements hard to read and your website will fail accessibility tests.
Will allow you to check the contrast of colours used on your site to see if they pass or fail accessibility tests.