"Faustino Dina" wrote ...
What means "balanced"?

It means "symmetrical", "push-pull", "differential", etc. where a pair of
wires carries the signal in opposite polarities. Any outside interference
will affect both wires equally, and the receiving circuit ( microphone
preamp, speaker, etc.) will cancel the "common-mode" noise.

I read somewhere that "balanced"
(cables) are beter than unbalanced ones,

In many cases. But there are tradeoffs, so it is not universal.

but I suppose the amplifier should output "balanced" signal....???

The vast majority of power amps (particularly those without output
transformers) have one side of the output grounded.

However, in most cases the wiring to the speaker, and the speaker itself are
completely "floating" (i.e. not connected to anything else). This provides
as much of a "balanced" circuit as you need for intereference-resistant
speaker wiring.