Contrast Ratios

I’m on step 46 of building a quiz and it’s about colour ratios. I struggled for a while, it was asking for a 7:1 contrast, how am I meant to know what a 7:1 contrast is?
I’m basically asking if anyone has any tips or advice on learning more about this as although I’m great with colour palettes, clearly I need to learn about contrast and ratios.
I found a website that seems to be helpful? I’d appreciate the help! Thanks!