This feature is disabled by default, but you can enable it on your mobile phone or tablet, by visiting the settings page, tapping "enable" in the "handwrite" section and then saving the settings.
"Say you're standing on a busy street corner, in a bumpy taxi ride, talking with a friend, or sitting on the couch with your tablet. Handwrite enables you to search by just writing letters with your finger most anywhere on your device's screen — there's no keyboard that covers half of the screen and no need for hunt-and-peck typing," suggests Google.
While this feature could be useful for short queries or popular queries that only require entering a few letters and using Google's suggestions, it's more efficient to use the virtual keyboard instead of writing each letter.
Google's Handwrite feature works for iOS5+ devices, Android 2.3+ phones and Android 4.0+ tablets and it's available for 27 languages.