Glissando is a fancy way of saying that you are playing all of the notes of the white keys in quick succession. You have heard it in some of those rockabilly songs. Here is the proper way to do it. Coming down the keys, use the back of your thumb. It will hurt a little bit at first, but as you get used to it the pain stops. Coming up the other way you use the back of your middle finger.
So now that you know how to play it, where do you put it in a song? Well, you've got to feel that. It can be used anytime you are approaching a chorus, or building up to something, or really anytime you really just want to dig in there and rock out. Just be sure not to use it too often. Any embellishment that is played to often can ruin an otherwise good song.
Next, I recommend you check out the song lesson on How To Play Happy Birthday on Piano .