Take two people Sam (average human) and Jack.
Jack says to Sam "I'm going to show you some skittles (or other object) and I want you to tell me as quick as you can how many I'm holding." He opens his hands and there are three. Sam can instantly tell there are three. Then Jack shows five to Sam, who takes a little longer, as he knows there are around 4/5 but can't tell so has to count.
As the number of skittles increases, Sam takes longer and longer to answer to the point where he needs to count the skittles one at a time to be sure, and sometimes to even recount them.
Why is it that we can instantly if we are looking at three maybe four objects but any more than this we cant? It gets harder and harder to estimate?