Users with poor visibility can use tools within their browsers or operating system to increase the size of fonts to make text easier to read. Using relative font measurements allow the text to adjust and flow properly based on a users settings. Using absolute font measurements on the other hand will fix the font size making it harder for these users to be able to read text properly on your website.
How can I resolve this issue?
Use relative font measurements such as em or %
What topics do this checkpoint affect?
Accessibility (WCAG 2 Level AA, 1.4.4)
Can you explain how this checkpoint works?
The checkpoint examines the entire HTML source code of a page and reports if any absolute font size are used within the measurement tags.