Google Analytics works by adding a small piece of tracking code to your website. It then reports visitors, where they came from (Google, Facebook, adverts, typing your address), which pages they looked at and what they did.
The current version is Google Analytics 4, often shortened to GA4. It replaced the older Universal Analytics, which stopped collecting data in 2023, so older reports and advice may not match what you see.
Because it uses cookies, UK sites generally need to ask visitors for consent before running it, unless configured in a way that current ICO guidance treats as exempt. It is also separate from Google Search Console, which reports what happens inside Google's search results.
Analytics tells you which parts of your marketing actually bring enquiries, so you can stop paying for the ones that do not. Make sure the account is in your name; if an agency owns it, your history leaves with them.