The original meaning of this idiom was very different than it is today - and that could be a good thing! It is said that back in the day , the medieval one, hunters would go out with big sticks and hit the bushes while making loud noises. The purpose was to scare any birds or animals hiding in the bushes. In retrospect, was that really such a good idea?  I mean, depending on what kind of animal might be hiding in the bush - would you really want to startle it and have it possibly charge you?! Anyway... These days it means avoiding the topic at hand or the answer to the question.

The literal meaning of this phrase has to do with hunting methods back in the day (I'm talking way back).  In mediaeval falconry where a bird in the hand (the falcon) was a valuable asset and certainly worth more than two in the bush (the prey). These days the modern meaning of this phrase is  it is better to have an advantage or opportunity that is certain than having one that is worth more but is not so certain.