Toast appears as a popup window in the lower-right corner of the active spreadsheet with a title and message. To create a toast dialog, edit or replace the greeting function as follows:

function greeting() {
  SpreadsheetApp.getActiveSpreadsheet()
   .toast("Hello World!", "Greeting");
}

Now if you click the button, then a toast dialog will appear as shown in the following screenshot, and it disappears within 5 seconds (the default):

You can include a third argument, that is, timeout seconds, in the toast method. This means how long the toast will be visible for. Put a negative number if you want it to show up forever. For example, toast("Hello World!", "Greeting", -1).