How to understand TTFB and target TTFB

Web Performance Discuss Test Results
1

How to understand TTFB and target TTFB, How is calculated TTFB for a web page? and the target TTFB for a web page?.

Example:

First Byte Time (back-end processing): 76/100
585 ms First Byte Time
354 ms Target First Byte Time

And What is the relationship with the grade and the value (76/100), the grade was “C”, and Why 76/100?

I can not found the formula for those values?

Thanks.