You can find documentation for the Reactome data model here.
Sidebar on the left shows the hierarchy of Reactome classes. The number of instances of this class is shown in square brackets and is hyperlinked to a page listing all instances in this class.
The main panel shows attributes of the selected class. Own attributes, i.e. the ones which are not inherited from a parent class are indicated in colour.
'+' in 'Cardinality' column indicates that this is a multi-value attribute.