Insertion loss, or S21, is a measurement of the loss of your signal strength from one port to another.
So, if your cable provider is delivering a 1mW signal to your house, how much of the signal is actually getting to your TV?
Because, if you have too much loss then you won’t get a good signal.
There are other factors at play - but, today let’s ignore cable loss today and just look at the splitter.
Let’s go back to college for a minute and remember our basic Circuits class. This a 2-resister splitter, so you would expect the loss to be 6dB.
You can see in the video that the loss is, in fact, slightly more than -6dB due to some loss from the cables. Just as expected!