Lets take a look at what happens in most cars (not all). The bulb in the "tail light" has two elements. One element is for the running lights or tail lights. This element is always on when your light switch is on. The second element in the bulb is used by the signal switch and also the brake switch. The second (and brighter) element comes on when you turn your signal on to turn left or right. That sounds simple enough.
Now, when you put your brakes on, the second elements (the brighter ones) in both of the right and left lights come on. The electronic circuit sends power to both signal lights at the same time and both left and right lights come on. This circuit also controls the signal light and the brake light together, so that you can signal and turn while your brakes are on. So, count the wires and you have 1. tail, 2. left/stop, 3. right/stop, and 4. ground. A total of 4 wires.
Now lets take a look at how a motorcycle works. A motorcycle does not have this electronic circuit to control the brakes and signals like that. Most motorcycles have one centrally located brake light and two signal/tail lights. One on each side of the bike. The tail/brake light is usually a dual element bulb. (2 elements inside one bulb)
When the brakes are applied, only the brake light comes on. When you signal a turn, only the signal light comes on. That sounds simple enough. A motorcycle has one wire to the right signal, one wire to the left signal and one wire to the brake light and one wire to the running/tail lights.
Count left, right, brake, tail and ground and you get 5 wires on the motorcycle, controlling the 3 lights on the back. (left,right,brake/tail and ground)
If your trailer has only two lights on the back, then you require a converter. If your trailer has a set of lights for your signals and tail lights and a separate set of lights for your brakes, then you do not require a converter.
An electronic converter will take the 5 wires found on a motorcycle and convert them to 4 wires for your trailer. When you apply the brakes, the power from the brake wire is split and sent to both signal lights on the trailer. It also prevents cross flow of power from one signal to the other when you turn a signal on.