Google Search Console
Google Search Console is a free Google tool that lets website owners monitor how their site appears in Google Search, spot and fix indexing problems, and view performance data such as impressions, clicks, CTR and positions. It also flags security issues, manual actions and incoming links.
In practice
Search Console is usually the first place to look when rankings go wrong: the pages report shows which URLs are not indexed and why, and the performance report gives real click and CTR data per query. For companies it is worth setting up as soon as the website launches, including submitting the XML sitemap. One common mistake is to open the tool only when something is already broken, instead of using it regularly to check crawl errors and new queries.