Once you have an SEO plan in place, it's important to be able to gain feedback- to be able to see what is working and what isn't. Is what you're doing having an effect on your website? Are you moving up the search results page? This is especially important if you need evidence of your efforts to report to supervisors or managers. One of the best ways to obtain this information is to use Google Search Console (previously Google Webmaster Tools).

The articles linked below will explain how and what metrics you can/should track for your website. The first article is the final chapter from MOZ's Beginner's Guide to SEO and provides a good foundation in tracking SEO data and metrics. The second is an updated article on Google Search Console, and is also the briefest. The third article, while written about Google Webmaster Tools, is very thorough and still applicable since little changed with the update to Google Search Console. 

Chapter 10 - The Beginner's Guide to SEO (Webpage)

Google Search Console: A Beginner's Guide (Webpage)

Beginner's Guide to Google Webmaster Tools (Webpage)