This thread is funny and lots of people here are hypocrites, no offense but when people post pictures of their E9X that is stanced and has poke everyone is all like "Awesome, nice, great stance, great looking car, I love how your car looks, what's your offset, what's your rim size and tire sizes"
Sorry but I just had to put it out there, but back to the topic. The OP is asking to explain what stretching means since he doesn't get it, he is not asking for an all out war about people's opinions on stretch.
Here's some information about this topic and for those ignorant people saying stretch and poke is "to ricey".
What is stretch?
"Stretch" refers to stretching a narrower tire over a wider wheel, the sidewalls stretch from the lip to the tread.
Why do stretched tires?
The whole thing started in Germany/Belgium, local laws required that the tire tread be within the fender line. Well that doesn't allow one to run wider or lower offset wheels, so they put narrower tires on the wheel to maintain legality.
When you would like to run really wide or low offset wheels and keep the tire tread at the fender.
When you would like to go really low and not rub.
Isn't this unsafe?
There hasn't been any soild evidence that running a stretched tire is anymore dangerous than a regular tire.
By the way stance, stretch, poke, staggered, modding, all this is a lifestyle.