To enable the feature, you simply download Google's "Smart Lock" app on iOS. Upon doing so, any attempts to log into your Google account will automatically send a verification request over to your Android device of choice.
It seems likely that there will be a few ways to verify a login request, but the default option appears to require you to hold down your Android phone's volume button for a couple of seconds.
This idea might seem a bit worthless to anyone who only owns a single smartphone (in either camp), but there are scenarios where it could be helpful. If you happen to prefer Apple-branded tablets, for example, but exclusively use Android smartphones (and carry both around with you), this makes logging into your Google account quickly and securely on said tablet just a bit more convenient.