Resource are typically things like meeting rooms or projectors etc, i.e. things that are used durings appointments. Resources can be booked for appointments to ensure that a resource is not double booked.
You can create and edit resources via the settings which are explained here.
To see available resources you'll need to access the Calendar module and click on the RESOURCES > OVERVIEW menu.
Here you'll get a Gantt-view of the different resources created in your solution and see what resources are booked and available.
You can book the resources while creating an appointment