Bindable Events are a way for UMG to mimic the behavior currently used by Slate which needs a single handler to tell it if the Event was handled. You can bind a function in your Widget Blueprint to the event from the Details panel under the Events section (shown below indicated by the yellow arrows).
Indicated above by the yellow box, some Widgets supplement Events through the handling of Interaction. For the example above, in addition to the OnClicked Event for a Button Widget, you are able to specify the handling of click events through setting the Click Method or Touch Method. You can also specify through the IsFocusable option if a button should only be mouse-clickable and never keyboard focusable.