A really quick example may help you out.
As a baseball fan, I am sure you don’t think it’s a big deal when a player hits .400 in a game, i.e. goes 2 for 5 in a single game. I am sure you’d be impressed if that same player hit .400 for a season, ie he collected 200 hits in 500 AB. That’s because 5 AB is not a big enough sample to tell us anything about a player’s true talent level. 500 AB however is a much bigger sample and thus a better reflection of a player’s true talent level.
The bigger the sample, the better reflection of a player’s true ability.