On your Android device, go to System Settings, Developer Options, turn them on, and be sure that "USB Debugging" is allowed.
On most devices running Android 3.2 or older, you can find this option under Settings > Applications > Development.
On Android 4.0 and newer, it's in Settings > Developer options.
Note: On Android 4.2 and newer, Developer options is hidden by default. To make it available, go to Settings > About phone and tap Build number seven times. Return to the previous screen to find Developer options, including "USB Debugging".