Copper of two different nets touching, so two wires become one. Signal corrupted, or board dead if power meets ground.
How it looks
Real geometry with an illustrated fault: this short does not exist on CORE3588
Real CORE3588 copper (the two parallel grey tracks are the USB data pair, 0.12 mm apart) with an imagined fault drawn in amber. CORE3588 itself passed DRC with zero shorts.
1Illustration: a speck of copper bridging the two wires of the USB pair. On the real board there is none; this shows what a short would look like.