Meaning of high audit risk

It means there is something on your return that says you are more likely than the average person to get audited.  There is not an actual number to say, you are 77% likely to be audited, it is just saying it is a reasonable chance it will happen.

I'm retired and never had a high audit risk. How can I find the cause?

In the event that you are audited, the IRS will reach out to you; you may review this helpful resource on tax audits @kdstengle.