I'm new to electronics so please forgive my ignorance.
I have a motor shield connected to an UNO. I have two DC motors connected to the shield. The Arduino is drawing power from the USB port.
I have a battery holder (on a 2WD Turtle platform) that has 5x1.5V AA batteries. What I want to do is connect he batteries to the motor shield but I'm frightened about wiring it up. Where do I connect the red and black wires of the battery pack? I also read somewhere that I need to connect the UNO and the shield to a common ground. What does that mean?
Motor shields usually take a relatively high voltage on their main power pins. 12V or more. Some will feed this to the Arduino's raw power pin but some won't, because more than 12V will be bad for the Arduino. There may even be a "jumper" on the board to allow you to choose, by connecting the two jumper pins together.
Motor shields almost always have a pair of terminals labeled "Motor Power" or something to that effect because it is not a good idea to run a microprocessor off a power source that is driving motors , due to the electrical interference. The battery pack should be connected to the motor shield motor power screw terminals and the motor shield GND and the arduino GND need to be connected or the motor shield will not recognize control signals from the arduino.