Page speed score is an excellent indocator of the website's performance. The higher the score, the better.
However, it's important to keep in mind: page speed score is just an indicator. It's a useful and important tool, but obtaining the 100% score on a real-life website is almost imposible. But more importantly, it usually is unnecessary also: the end-goal is to get a fast-loading website and a visitor who gets the requested page in a few seconds.
How to obtain a page speed score
The most popular tools are;
This article over at WPBeginner.com gives a nice overview of the most popular online services.
Is it complicated to apply the suggestions from a page speed score service, given after the test?
Please note that the suggestions, given by page speed score services after the test, could be pretty technical, and sometimes overwhelmingly cryptic. In that case, it's probably best to hire a professional WordPress developer or an agency.
But again: the suggestions don't have to be absolutely followed. The end goal for the website is to provide a pleasant and unobtrusive experience for the visitor. If the website already achieves that, improving the page speed scrore probably could be very low on the priority list.