You can read the basics of the module on the project page.
The Simple Field module provides a simplified UI for adding fields and instances, and it also removes some of the complexity of the standard Field UI. One of the important things that is removed is the ability to assign different instances of a field different settings.
When first installing the module, you, as a site builder, should decide what type of users will be using which features. You have a choice in that you can limit users to only create new instances of pre-existing fields, or you could allow them to actually create fields themselves using the simpler interface. You'll also need to decide which types of entities you want to take advantage of the UI, and which ones are admin-only and can keep the standard Field UI module interface.
As a site builder, you will also have to decide what types of fields should be available to different people. Some types are provided by default, but it is likely that you will need to provide more.
Currently, the only way to define new types is through code. One big TODO is to make a module to allow admins to create new types. This will only go so far though, because some types that require custom user input when the field is created, which will be tough to define through a UI.
To define simple field types in code, there are two hooks that you can implement. They are heavily documented insimple_field.api.php. As a quick explanation, hook_simple_field_type_info defines types along with any default settings for the type. Basically this hook allows you to hard-code Field, Widget and Instance settings. Second, hook_simple_field_type_form allows developers to provide form elements to allow users to specify other field settings. Any other field settings will be merged in with default values provided by the field itself.