You can pull up Now on Tap the same way you used to launch Google Now, such as holding down or double tapping the home button (depending on the device). First time you do this, you'll be prompted to turn on the feature.
Once activated you can get additional information along with what Now on Tap determines you would want to see. For example, if you are on a web page, or listening to a song, press and hold the home button and you'll get the info on the song. Press the microphone button and ask a related question, such as a year the artist was born, and you should get the answer.