Buttons are the execution of any action. For example, you can create buttons to execute: Copy to clipboard, Send an email, Open a link, Link to page and others.
To add a button, you need to go to Visual Builder and simply drag and drop the Action to the page, specify the button name and action type. More information about types of Actions.
For example, you want to add a button that will send emails. To do this, select the type of action - Run Query.
More information about Actions.