Basically, it's when someone doesn't want to admit that they don't know the answer to your question. They don't want to say "guess", so they say "approximate decision" instead. So, basically... A guess.
What Does Approximate Decision Means?
Approximate decision making encourages people to make decisions quickly even when not all the facts are known. It is a counter to "analysis paralysis" which is when someone requires more and more detail before getting on with the next step, the result would be that they miss opportunities or just spend far too long. So the purpose of Approximate decision making is to make effective decisions as soon as possible. Links with ideas such as the 80:20 principle etc.