WebDriver CssSelector is an alternative means of searching for an element on a webpage. As opposed to Xpath which scans through the html content of a webpage CssSelector looks through cascading style sheet (css) instead.
WebDriver CssSelector Video Tutorial
CssSelector is also subject to the same fragile nature like Xpath. If the cascading style sheet changes then any tests which relies on the style sheets then they will begin to fail. It is important to remember that any changes to the cascading style sheets will most certainly be out of your control. This may result in tests which fail overtime.