The Activity Explorer provides an easy alternative to find out, how activities relate to your process.
This is how an "empty" activity explorer looks like:
Activity Selection
Start with selecting one activity for your analysis.
Click on Select activity (or on the selected activity, if one activity is already selected here).
The following window will open:
Click on any activity to select it.
The search field might help you to find your desired activity.
Relation Selection
In the next step, we need to set a relation.
The relation defines the condition, that a case needs to have to be added to the accumulated Activity selection, that is displayed in the Activity chart.
By default, Cases flow through is selected.
Activity chart
The Activity chart compares the component selection (based on the settings that have been made in the Activiy selection and the Relation selection, see above) with all other available cases (which might be restricted by global Selections).
If you wish to apply a global selection based on the this selection, use the small 
icon in the upper right part of the activity chart.