netstandard is a standard, a specification.
In the same way that HTTP is a definition of structure and behaviour that app developers can implement to be compatible with HTTP servers, NET Standard is a standard for classes that can be implemented by platform developers so that all libraries targeting the standard are compatible with it.
For the five-year-olds, think of it as the shape of an electrical outlet. Here in the Australia, we have the three pronged outlet. You could think of the house as the platform, who, by choosing to implement the standard outlet, is then able to work with just about every appliance (library) made for Australia.