The Event Manipulator action has two options, it can stop the 'propagation' of the event, and also it can prevent the default behaviour of the event.
Propagation of the event can be roughly defined as the event being passed down DOM elements, and being fired by each of those elements. For example, if a DIV is above another DIV, and it is clicked, both DIVs will receive notification of the click, and both can fire an event. You can prevent this by stopping propagation.
Some events have default behaviours. An example would be touch scrolling on an iPad, it's default behaviour is to scroll the page. If you want to stop this behaviour, and make the event do something else, you may use the Prevent Default option.