Attributes values are the different variant from a Attribute. Please refer to Attributes for more information.
Following the example from Attributes,
Color attribute can have various values. Black, White, Red, Green......
In order to define these values, they can be added in the Attributes Values.
Go to Sales -> Configuration -> Product Catogories and Attributes -> Attribute Values
Attribute Values can be added easily by first selecting the Attribute, then the enter the Value you need.