The fine art of expectation guessing
While we aren’t very good at estimating things absolutely, it turns out we are pretty good at estimating things relatively.
Sizing stories relatively means not worrying about exactly how big a story is, and worrying more how this story's size compares to others.
This style of estimation (relative over absolute) forms the corner stone of Agile Planning. By sizing our stories relatively, and feeding actuals back into our plan, we can make some really accurate predictions about the future while based on what we've done in the past.
To learn more watch this short video on Agile estimation.