system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is impedance?

I'm currently trying to get the hang of electronics and sound production and I come acrosse "impedance issues" from time to time... For instance, "a preamplifier might load the microphone with low impedance, forcing the  microphone to work harder and so change its tone quality" or a contact mike having a low amount of bass because its impedance doesn't suit the amplifier...  

Can anyone help me?
gpt
Impedance is in general, the electrical resistance, but the why can vary a bit.

For what you want to know, it's two things, with microphones you need to understand that they typically put out very little power, therefore you want a high impedance pre-amplifier (one that draws very little current from the microphone). This is because with things like a microphone it may physically stop working under high load. Typically that's what a pre-amplifier is for, they have a high impedance input and low impedance output, they don't draw significant power from your microphone but can provide moderate power to drive your amp and normal equipment (so you can plug lots of equipment into your pre-amp without reducing the signal out of your microphone).

With speakers it's a bit different, amplifiers are designed with an output impedance that needs to be matched to the speakers. Basically they have a certain max voltage, and a max current, and thus to get the rated power the speaker needs to have the right impedance such that at the max voltage it draws the max current. Failure to do that means the rated power doesn't go into the speaker. With some amplifier designs it may also mean the amplifier is damaged as that missing power actually goes into the amplifier and may blow the amplifier.

Now as what impedance is, it's how we measure the resistance to current flow when an AC signal is flowing. Many things have frequency dependent resistance, so impedance is the resistance at a specific frequency. For speakers, typically the nominal impedance (the number on the sticker) is the lowest impedance over the entire audio range, but it varies. For low frequency this is important because the speaker may drop below the nominal impedance below the "audio range", and audio range may be arbitrarily defined.