Control threshold is often described as the limit of the project. For example, there might be a threshold in the finances that will be used for a certain project. This means that it can only reach up to that certain amount and the finances will not go beyond that anymore.
This may also refer to the quality of the product before it will be released to the public. There are times when the quality control is really bad and it can be the downfall of the company. Making sure that there is a threshold will at least allow some control over various factors that the company finds important.