Descriptions of output impedances of musical gear without numbers:
Guitar output jack: Medium to High (passive electric)
Mic output jack: Low
Keyboard/Sampler/Drum Machine outputs: Low
Amplifier output jack: Extremely low
Descriptions of input impedances of musical gear without numbers:
Guitar input: High to very high
Mic input (on mixer, XLR in): Medium
Keyboard/Sampler/Drum Machine ("line input"): Medium
Speaker input terminals: Very low
Here's the exact same list, but with numbers:
These are the output impedances of musical gear usually:
Guitar output jack: 5,000Ω-500,000Ω as frequency goes up (passive electric)
Mic output jack: 200Ω
Keyboard/Sampler/Drum Machine outputs: 1000Ω
Amplifier output jack: <1Ω
These are the input impedances of musical gear usually:
Guitar input: 1,000,000Ω
Mic input (on mixer, XLR in): 600Ω to 100,000Ω (ranges based on design)
Keyboard/Sampler/Drum Machine ("line input"): 10,000Ω-100,000Ω (ranges based on design)
Speaker inputs: 2Ω to 16Ω
Comparing the matching output to input looks like this:
Guitar: Med/High -> High/Very High
Mic: Low -> Medium
Line: Low -> Medium
Amp to speaker: Extremely low -> Very low
In each case we see the source of the sound (guitar, mic, keyboard) is lower impedance than the load, or next stage's input.
Amplitude and Gain
What can really confuse the issue is the amplitude and gain that inputs are expecting.
Our examples again, now with some average levels... I'm leaving out important technical details (like "rms" vs. "peak-to-peak") to get your oriented:
Voltage amplitudes... how "big" the signal is coming out of the device. This is just a rough guide, an assumes whatever "volume" controls are turned up to max or near max:
Guitar: 0.1V
Mic: 0.01V
Keyboard: 1.0V
100W amplifier: 30V
Line level, without getting technical, can be thought of as around 1V.
An amp driving a speaker ("power amp", "powered speaker") needs something near line level to get a good, audible signal through the speaker.
Going back to our voltage examples, the different sources need different levels of boost to get up to that 1V level.
Guitar: 0.1V, needs about x10 or more of a boost.
Mic: 0.01V, needs about x100 or more of a boost.
Keyboard: 1.0V, may not need any boost.
100W amplifier: 30V, the amp takes that ~1V, and not only boost the 1V x30, but it also adds current gain so that there is a power boost (power = voltage x current) as well.
Sources (guitars, mics, keyboards) have varying output levels, so we need variable gain controls. Sometimes a gain of x1 is needed, and sometimes we like a gain of x1000.