A short print — or SP — is a card that was produced in smaller quantities than the standard base cards in the same set. Fewer copies printed means fewer copies available, which means higher demand and higher prices.
Short prints can be subtle. Sometimes the only way to tell an SP from a regular base card is a tiny difference in the photo, a variation in the fine print, or a different card number format. Savvy collectors learn to spot SPs quickly, because pulling one without realizing it means you might trade away something valuable by accident.
There are also SSPs (super short prints) and even SSSPs (super super short prints) — each level rarer and more valuable than the last. The deeper you go, the harder the hunt.