You should read the documentation about DesiredCapabilities. There is also a different page for the ChromeDriver. Javadoc from Capabilities:
Capabilities: Describes a series of key/value pairs that encapsulate aspects of a browser.
Basically, the DesiredCapabilities help to set properties for the WebDriver. A typical usecase would be to set the path for the FirefoxDriver if your local installation doesn’t correspond to the default settings.