Google Tag Manager(GTM) is a tag management system that allows you to quickly and easily update tags and code snippets on your website or mobile app, such as those intended for traffic analysis and marketing optimization. You can add and update AdWords, Google Analytics, Firebase Analytics, Floodlight, and 3rd party or custom tags from the Tag Manager user interface instead of editing site code. This reduces errors and frees you from having to involve a developer when configuring tags.
A tag is a snippet of code that sends information to a third party, such as Google. If you don't use a tag management solution such as Tag Manager, you need to add these snippets of code directly to files on your website or mobile app.