What electronic component is this? It says "K103M" on top
What electric component is this?

It is a capacitor.

My Multi-function Tester says it is a capacitor with a value of ~8958
pF (=8.9 nF)
It says &quot;K103M&quot; on top of it. Could this mean that it should be
10*10^3 = 10000 pF?

It is a 10nF capacitor, the K and M I believe are the voltage and tolerance, I believe that the letter at the beginning is the voltage and letter at the end is tolerance although I could be wrong
Depending on tolerance, 8.9nF could be within the tolerance of a 10nF capacitor. If I am correct with the tolerance being the letter M, that is ±20% if I recall correctly, which would mean this capacitor is indeed within tolerance.
You can look for capacitor printed codes on Google to double check values and look more into the voltage and capacitance tolerances.