What does this mean?

[daisy]

If someone says:

your {insert mm} has barrel distortion

What does that mean exactly?

 

[Big Mike]

Barrel Distortion
Some focal lengths are more prone to different types of distortion. Usually, better quality lenses have less distortion.

 

[The_Traveler]

http://en.wikipedia.org/wiki/Image_distortion

 

[ksmattfish]

It means they are probably a soulless automatron more concerned with gear, process, and rows of technical numbers than art. Sure they can spot barrel distortion at 20 feet, but ask them what makes a photograph beautiful, and they are stumped. Don't worry about it; you can see barrel distortion just as well as they, but if it didn't bother you before you knew what it was called, it shouldn't bother you now.

Depending on what you are doing, lens distortion may or may not be an issue. The idea that there is such a beast as a distortionless lens is silly though. Look at a photo of someone taken with the greatest lenses ever designed; do you think in real life that they are 2 inches tall and flat?