Threshold value

I was wondering what unit the threshold values are in. For instance, in the Knock tutorial, there is a section that says:
const int threshold = 100;

What does the 100 mean?

In that context it refers to the analog voltage level through the ADC which ranges from 0 to 1023. So with a 5V reference, 100 would be about 10% and 0.5V.

In that context it refers to the analog voltage level through the ADC which ranges from 0 to 1023. So with a 5V reference, 100 would be about 10% and 0.5V.

To add on to that:

In that particular example, it simply means that it won’t register as a “knock” unless the analog reading exceeds 100, which, as the previous poster said, was about 0.5V.