Google Tag Manager is a platform that manages tags. Tags are bits of code that take the actions of users on your site and pass those through to GA4 for reporting.

The container code sits on every page of your site. As tags are built in the back end of the platform the code on the site pulls them through so they can do what they need to do.

These tags only work when their trigger is activated. A trigger in GTM is a small set of rules that tell the Tag Manager when a Tag should be active. Tag Manager comes with a set of prebuilt Tags and Triggers that help with basic set ups, more often, a more complex set of Tags and Triggers are needed to accurately pass the right data through to GA4.

Tag Manager is a way to manage tracking across as many sites as you need, set up properly it respects users tracking consent preferences and only tracks what you need, as a business, to track.