Do you have the SmartDashboard running? The SmartDashboard is not the default dashboard or driver station. You can get it
here. It's intended to be a replacement for the default dashboard, and easier to customize.
The SmartDashboard displays data based on the key of the data. The first parameter of putData is the key, the second is the data. If I wrote putData("Gyro Heading", gyro.getAngle());, a field would be created called "Gyro Heading" with the current value of the gyro. Subsequent calls to putData with the key "Gyro Heading" would update the same field instead of creating a new one.
The 123 was just a sample value to send.