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?
The definition is very clear:
Level AA: For Level AA conformance, the Web page satisfies all the Level A and Level AA Success Criteria, or a Level AA conforming alternate version is provided.
Every single rule in the guideline has a level of conformance it belongs to. If you fulfull all requirements marked with AAA, your site has the highest compliance.
To answer your comment above. The background and foreground colours pass or fail on a specific algorithm, which is different for AA and AAA. An overview is given below:
AA = Contrast ratio of at least 4.5:1 between background and foreground. 3:1 for large text (over 18 point or 14 point bold)
AAA = Contrast ratio of at least 7:1 between background and foreground. 4.5:1 for large text (over 18 point or 14 point bold)
With regards your question does it mean colour blind people have difficulties, that depends on various factors. Consider checking your site on a colour blind simulator such as http://www.vischeck.com/. Personally I think the WCAG guidelines are too limiting at the lower contrast levels. Also they have no limit for high contrast. Too much contrast can often be an issue for users with dyslexia I believe.
If accessibility is core and you have budget find a way to test with users with various disabilities, as colour contrast is only a small part of accessibility. There are companies that offer these services I believe.