Fluent NHibernate has two mapping styles: Auto Mapping and Standard Mapping (aka non-auto mapping, or just fluent mapping). This page is about the latter.
The GettingStarted: First Project example uses fluent mapping for it's mappings, so you should refer to that if you've not done any work mappings before; if you're looking for some more complex examples then this is the place.
You can read about the ClassMap<T>, mapping properties, mapping relationships and more from the right hand menu.