There area a number of tools depending on the detail you want to measure and your budget.
Google Analytics is easy to install - free to use - and gives decent amount of analytics that you can use to measure a lot of variables to find out your retention (how much time a user spends on a page, bounce rate, unique vs returning visitor etc.)

On the more professional end of the spectrum, there are solutions like ahrefs, semrush... These are somewhat more focused on SEO and search analytics, however they can generate great insights. I have personally used ahrefs, and although I find it a tad on the expensive side, it can be worth the money if you can utilize it.