Here’s a WordPress optimization guide that we made to help WP users:
https://gtmetrix.com/wordpress-optimization-guide.html
The “Use efficient CSS selectors” rule generally refers to how easy it is for your browser to parse for the html element/class defined in the CSS.
A style like ul li { color:red } would require the browser to search for all li’s, then for all the ul’s that are parents of those li’s. To be more efficient, you would only need to declare li { color:red }. (However, there may be a requirement to have the descendant selector for specificity.)
I generally wouldn’t worry too much about this rule. It’s a “Low” priority and for an existing site, is a lot more work fixing than it’s worth.
Unless you have a really large page with lots of interactivity, it’s not going give you much benefit.
