GeoServer GeoSciML WFS
By default, Geoserver's WFS server is capable of conveying only simple features. Basically, a simple feature has some number of attributes, and each attribute has a value. Think of it like a flat table, where each row represents a feature, and each column is one of the feature's attributes.
However, in the features defined by the GeoSciML schema are complex - that means that a single attributes might have multiple values, or the attribute might actually be another feature altogether! As an example, think of the description of a geologic unit as a complex feature. One of the attributes of that feature might be simply age. That geologic unit may have an age of depostion, and who knows how many different ages of deformation and metamorphism. So our geologic unit has multiple values for its age attribute. A geologic unit might also have an occurence attribute, again with multiple values, with each occurence representing a polygon on a map where we would find rocks of that type. In this case, we see that the value for the occurence attribute is actually another feature altogether - a polygon that might have its own set of attributes!
Through the application-schema (or app-schema) extension, Geoserver becomes capable of becoming a complex-feature WFS, and therefore, capable of serving GeoSciML features. Better yet, a lot of the development of the app-schema extension was done with GeoSciML specifically in mind! Combined with the easy-to-use Geoserver interface, GeoSciML in Geoserver is a winning combination.