Once you have selected a level and possibly a scope, the tabs at the bottom of the levels table will contain any tangles and clusters in the corresponding dependency graph. Select any of these and the dependency graph will display the corresponding sub-graph.
The "main cluster" is almost always the main bulk of your application code. The other clusters are called "orphans" and may be unreachable "dead" code (though not always, e.g. if this is a library component).
The #Parents column indicates the number of parent items (packages, folders, ...) spanned by the tangle or cluster. The negative effects of tangles are more pronounced if they span more parent packages. For example, a tangle that is localized within a single parent is probably less disruptive than a tangle of the same size that spans multiple parents.