Features are a product's intrinsic characteristic: they remain the same throughout the product's variations (or "combinations").
You should think of features the same way you think of product attributes, with the notable difference that you cannot create product variations based on features.
The difference between an attribute and a feature depends on the product itself. Some products might share the same property, one being an attribute built for building product variations, another one being simply an invariable feature.
For instance, a customer can buy an iPad variation based on attributes (color, disk space) but not on features (weight, size).
Likewise, another shop might sell t-shirts variations based on attributes (color, size, gender) but not on features (weight).
In your shop, features are displayed in a table, giving additional information about the product; while attributes make it possible to select among product combinations.
Features are configured on a per-product basis, from the "Product" page under the "Catalog" menu, but they must first be registered on your shop using the tool on the "Features" tab under the "Attributes & Features" page from the "Catalog" menu.
You can set the features' order using the arrows in the "Position" column.
The "Actions" column gives an Edit action, a Delete action and a View action which you can click to display the available values for this feature. From there, you can sort, edit and delete values if you need to.
Click on "Add new feature". A very simple form appears. This is where you name the feature itself: for example, the types of headphones provided with a music player.
Give a name to this new feature, and save your new feature to return to the attributes list.
Click on "Add new feature value". Another form appears.
Fill out the form:
...and save your feature value. You can add more values for the same feature type by saving your changes with the "Save and add another value" button.