PHP workers are processes that execute your PHP code simultaneously. All shared cPanel plans come with 2 PHP workers at first, and we've found this limit to be sufficient for basically all sites.
If your site received lots of traffic though and processes PHP slowly, you may need this limit raise. Please contact our support team and we are happy to help. Keep in mind, PHP workers count against your resource usage (CPU and Memory) so raising this limit to an abnormally high amount can actually slow down your website.