Pin stripes (also spelled pinstripes) are a pattern of very thin stripes (often as thin as a single pin/thread, hence the name) that run vertically.
The pin stripe has come to be associated most commonly with the pinstripe suit, a traditionally conservative form of business attire. However, pin stripes can be found in many other pieces of clothing as well, from blazers to socks.
Pin stripes are a simple but effective pattern that breaks up the monotony of an otherwise uniform piece of fabric. Because pinstripes run vertically, from top to bottom, they help to draw the viewer's eyes vertically as well, which makes them a good design choice for people who are shorter or overweight, who don't want to draw attention horizontally.
Here are some examples of pin stripes in action: