Here's a cheeky bit of code that'll help you to dismiss an alert box. If you don't know what an alert box is then head to http://lazyautomation.co.uk/lazy4.html and click the 'Click Me' button to see one in action.
Now let's automate it! The following code will open a browser and navigate to the same test page, click the button and then dismiss the alert box. Then as an added bonus it'll also grab the text from the alert and output it to the screen just in case we need to know what it actually said.
require "watir-webdriver" b = Watir::Browser.new :chrome b.goto 'http://www.lazyautomation.co.uk/lazy4.html' b.button(:id, 'button1').click sleep(3) #I've put this sleep here just so you can actually see the alert before it gets dismissed puts "Alert displayed saying: " + b.alert.text b.alert.ok