Fixed pitch means that the main rotor blades always have the same angle, and it is the throttle, or main motor speed, that controls the heigth. Higher end helicopters uses collective pitch, meaning that the mail rotor can run at a constant speed, and the angle or the blades is changed in order to contol the heigth. But this is more complicated and expensive, and is not good for begineers.
coaxial means that there are two main rotors, that rotates in different directions. With this mechanism, there is no need to have a vertical tail rotor that compensate the torque induced by the main rotor. Instead, the two main rotate at the exact same speed, to keep the helicopter heading, or they run at different speed in order to make the helicopter rotate one way or the other.
Then, the only thing that is needed is something to give some forward or backward speed to the helicopter. This is accomplished by having a horizontal tail rotor. This one can push the tail up, in order to lean the helicopter forward and make it gain some speed. Or it can puch the tail down, in order to lean the helicopter backward and reduce the helicopter speed or go backward.