A range around a measured number that shows how precise the measurement is. Pull rates come with a 95% confidence interval: TCGplayer's 4.88% Special Illustration Rare rate for 30th Celebration carries ± 0.74 points, or 4.14% to 5.62%. The narrower the range, the more precise the rate.
For new collectors
A 95% interval doesn't mean a 95% chance that the true rate sits inside it. It means the method, used on many samples of the same size, catches the true rate about 95% of the time. More packs opened make the range narrower.