Google Search Console is a free tool from Google that shows you how your site is doing in search. What people typed before they clicked you, which pages are in the index, and what Google could not read. It is the only place this information comes from first hand.
The three screens worth your time
Performance shows the searches people used and how many clicked. This is where you find the wording your customers actually use, which is often not the wording you use.
Pages, sometimes called indexing, tells you which pages are in Google and gives a reason for each one that is not. Sitemaps is where you submit your sitemap and see whether it was read. Everything else can wait.
Setting it up
Go to Google Search Console and add your site. You prove it is yours, usually by adding a TXT record in your DNS or by uploading a small file. A domain wide verification by DNS covers every version of the address at once and is worth the extra five minutes.
Set it up in an account you own. If it is verified only under a designer login, you lose the history along with the designer.
A worked example
A drainage firm looks at Performance and finds the top search bringing people in is not blocked drain at all. It is drain smells of sewage, which they had never written on the site.
They add a short page answering that exact question. Three months later it is the second biggest source of calls on the site. Nothing was guessed, the data was already sitting there.