A passive splitter does not require electric current for working. As it operates passively through transformers there is also a small insertion loss per connected load that, as long as the input audio has a correct line level, will not be a limitation in its use. It is a device that can be used anywhere without having to have a power outlet.
An active splitter requires electric current for its operation since it offers functions such as amplification of the audio output and monitoring of the levels by means of LEDs. It is a device, therefore, that requires a power outlet to operate.