Google Tag Manager is a free tool that allows you manage and deploy marketing tags (snippets of code or tracking pixels) on your website (or mobile app) without having to modify the code.
Here's a very simple example of how GTM works. Information from one data source (your website) is shared with another data source (Analytics) through Google Tag Manager. GTM becomes very handy when you have lots of tags to manage because all of the code is stored in one place.