I am explaining this, in the subwoofer you have a high-level input, if you have a stereo amplifier, you can connect the speaker system through the subwoofer - it will "steal" some signal from the speakers and amplify it. There is no amplifier for the front speakers.
It looks like this graphic from the internet:

With an AV receiver with active subwoofer output you don't need to use the high level inputs at all.