A Threshold value in DCC applications refers to the conditional value that is used to either trigger an evaluation or stop it. For example, if you are working with a sub-surface scattering (SSS) shader, you will see a Threshold setting in the advanced section; you can use Threshold to fine tune the evaluations within the same attribute group. Often you can achieve faster rendering results, with an acceptable loss in quality, by working with the Threshold value. Depending upon the effect, Threshold values can vary widely in range.
Threshold values are common in any image processing or compositing application or shader. In short, they’re everywhere.