Rc coupled amplifier

[audioguru]

what does "strong" mean here? Is "3.5k" big enough or small enough in your context? I guess, by "strong", you meant the signal source is some sort of constant "voltage" source, that it can impose a constant voltage drop on the load disregard the load's impedance. right? if this input signal is coming from a CD player or MP3 player or sound card lineout, can these signal be regarded
as "strong" enough? and why?

Most amplifiers have an input impedance of 10k ohms or more. The output impedance of an MP3 player or CD player is much less because it is a constant voltage so that there is very little signal lost.
Your circuit has an input impedance of about 3.5k ohms so some signal will be lost because the source impedance is not strong enough to drive the fairly low 3.5k ohms impedance.

Trying to understand this (simplified case), i am imagine a kind of hydraulic analogy of this....

The signal does not affect the power supply voltage because the power supply is very strong and does not change.
Each 10k biasing resistor has such a high value that it barely attenuates the signal.