DMX is based on a serial protocol. Meaning that the channels are transmitted through the cable no matter if a device is actually on the other end. To get the fixtures working properly, they need to “know” which channels they should obtain from this continuous data stream.
A LED Par that does Red, Green, Blue and White, will very likely demand 4 control channels. If you set the LED Par up to start at channel 1, it will interpret the following 3 channels as well.
Depending on the channel value, the according function is executed by the fixture. Typical this means that a value of 0 will represent blackout whilst a value of 255 is full brightness.