Beginners typically start with rockets that fly using A, B or C motors.
The letter at the start of the motor code denotes the total impulse or power rating. Each time you move up a letter the total impulse doubles. So a B motor is twice as powerful as an A motor and a C motor is twice as powerful as a B motor.
The first number in motor code is the average thrust of the motor. This number takes into consideration how quickly the motor burns. So for example an Estes C6 and an Estes C11 motor have very similar total power (total impulse) because they are both C motors. However the C11 motor burns faster, because it has an average thrust of 11 compared to the C6 motor’s average thrust of 6.
The final number in the rocket motor code (after the dash) is the time delay before the ejection charge is fired. The ejection charge will eject the parachute or streamer out of the rocket, so you want this to occur when the rocket slows down at its highest point in the flight (Apogee). A C6-5 has a 5 second delay and a C6-7 has a 7 second delay.
This may all appear a little overwhelming, but the good news is rocket manufacturers will provide you with a list of motors that work well in the rocket. So follow their advice and you should have a good flight.
Motors typically come in packs of 2 or 3.
If you want to get a deeper understanding of rocket motors codes then this Apogee newsletter or Apogee video provide lots more information.