In User Guide
In is an ExpressionEngine add-on. It does not have a user interface, and is intended as a development tool to be used in your templates. It allows you, as the developer, to Include template files (kind of like embeds, but earlier in the parse order), and to Insert files as early-parsed global variables (kind of like snippets, but using plain ol' templates).
It is an easy-to-use alternative to snippets. Unlike snippets, which are always loaded when EE runs, In only uses the templates when they are requested. Easy, performant, useful.