I don't know what is 'selected' means

there are somethings like ‘OnSelected’ or ‘set selected’.
I can’t understand the difference with ‘OnClick’.
how can I activate the selected states when I’m previewing.

For most widgets the selected state is only activated when you use the Set Selected action to put the widget into the selected state. You can also add special styles to the widget, including one for when it’s selected, by right cooking the widget and choosing Interaction Styles… from the contextual menu.

thank you. It was very helpful to understand.