It depends on the type of pop ups you're getting. If it's from within the browser, then it's just an ad. Ignore them or stay away from sites that have too many of them.
If you're talking about notification pop ups, then it's because an app is a little too overzealous with ads. Go to the notification shade and long press on the offending notification until it says app info. Tap on app info and it will take you to the app settings page. You can uninstall the app so it no longer sends those annoying notifications.