Apparently, when you move to another tab in your web browser, the tab you were originally using can detect that it has lost focus. A malicious website can take the opportunity to load new content in the tab, change its title and appear to be a different website.
When you return, if you try to login using your credentials, you could be handing them over to a malicious source.
Make sure to look at the location bar to ensure you are on the website that you think you are. If the page says "Paypal" but the location bar says "johnnyscashemporium.com" you've probably been victimized by tabnabbing.