Google Tag Manager (GTM) is a container that lets you deploy and manage tracking tags, pixels, and conversion tags through one interface without editing site code each time. It is the backbone of a maintainable measurement setup.
What it does
Instead of hard-coding every pixel and tag into the site, you install the GTM container once and manage tags, triggers, and variables inside it. That means faster changes, versioning, and one place to govern how your pixel, conversion tags, and analytics fire.
Client-side and server-side
GTM runs client-side in the browser and also as a server-side container, which is more resilient to blockers and cookie limits. Getting the consent order right (defaults before tags, via Consent Mode) is a common source of measurement bugs.