Most (if not all) Android devices have the feature to use "vibration", i.e. to let the device vibrate. If your question is related to that, this is your tag. Read on by clicking below 'info' tag.
"Vibrate" is used to call the users attention to certain events, as well as to give "haptic feedback". This includes (but is not restricted to):
- haptic feedback: e.g. to confirm input (having tapped a key on the virtual keyboard and the like)
- notifications: e.g. vibrate on incoming calls/messages etc., or on other events happening. This can be in addition to a sound played -- or, especially useful, if you've put your device into "silent mode" to not disturb other people, to not miss a notification though no sound is played.
Other uses are thinkable (if not even yet practiced), such as haptic feedback in games (imagine a car race, you're the driver, and you speed up your car/bike and have it "roar" not only by sound but by feeling as well).