Schemas, despite their technical sounding name, are a very simple concept. In construction, a blueprint is type of schematic. It's a pattern or set of instructions to follow when building something else.
In GraphQL, and in database development terminology where the term was borrowed from, the schema provides the same function.
Under the hood, the schema defines what types of data your content can support. Are they mostly strings and numbers? Do you have any images? Are any of the fields required? That's what the schema defines.
However, most people don't like writing schemas by hand, and that's where our beautiful CMS comes into play.
The schema window is an editor for two different kinds of data which are called a Types in GraphQL.
Models which are an Object Type.
Enumerations which are an Enumeration Type.
Again, this serves only to familarize yourself with the terms of the technology, you won't likely ever need to know the difference. We handle that for you.