Google Tag Manager (GTM) is a tool Google provides to easily place and manage multiple tags, triggers and variables on a website.
Tags are generally snippets of codes, either first-party or third-party cookies, that indicate something about a user on a website.
Triggers are the bridge between variables and tags. These triggers speak to events on the website that quite literally trigger a tag to fire.
Variables tell GTM what items to pick up and track. Variables can include scroll depth, button actions and more. By default, URL is a variable that GTM picks up without having to enable.
Google Tag Manager containers help by allowing the placement of other containers within, but is also widely used by Google for other parts of the advertising suite. Google Tag Manager is also used in Google Ads audience (remarketing) and conversion tracking codes, despite not using the explicit GTM reference in these (they use AW-), it is built onto the GoogleTagManager.com reference.