What is the difference between WCAG 2 AAA Compliant and WCAG 2 AA Compliant

I use a web app to check my site color contrast. Most of my color (Back/fore ground colors) are OK and I see YES inside textboxes except the one which refers to WCAG 2 AAA. What is it? What is the difference between WCAG 2 AAA Compliant and WCAG 2 AA Compliant?