Popup message box

Code:

Browser.msgBox("Hello!", Browser.Buttons.OK)

Another popup message box is the toast, which shows up at the lower-right hand corner of the spreadsheet (this only works with sheets):

SpreadsheetApp.getActiveSpreadsheet().toast("Hello", "This is the title");

If timeout seconds is set to -1, it means that the toast will be there forever.

Create a modeless dialog that does not prevent you from doing other things:
function showDialog()
{
  var html = HtmlService.createHtmlOutputFromFile('hi');
  SpreadsheetApp.getUi().showModelessDialog(html, 'This is the title')
}