A USP is a ‘Unique Selling Proposition,’ or a ‘Unique Selling Point.’
In other words, it’s the reason why someone should buy from you, and not from another vendor in the same space with a similar product. Why use Slack and not Facebook at work? Why buy pizza from Papa John’s and not Pizza Hut?
Some businesses are the only one in their field. They exist in a marketplace of one, either because they’re gigantic or because they’re so innovative that nobody else is doing what they’re doing. But that situation seldom lasts.