"USP" is a marketing term.
The unique selling proposition or more commonly the 'Unique Selling Point' simply abbreviated to 'USP' is a marketing concept that was used as a theory to help understand a pattern among successful advertising campaigns, it dates as far back as the 1940s. It states that such campaigns made unique propositions to the customer and that this convinced them to switch brands.
Today it is used more frequently to identify what it is in particular about a product or a business itself which is different to a competitor, different enough to allow a sales advantage.
For example, if you sell one product and customers can buy this product from thousands of different merchants, why should they buy it from you? Let's say, everyone sells the product for the same price, £10.00 and you advertise it as £9.99 - you are the only merchant they can save a penny hence, it's a unique selling point however unlikely to make a difference!
Let's try something more realistic. Let's say you are a therapist specialising in reflexology. Now, there are many reflexologists so why would a person choose your services over others? You might review your own professional history and may find something that makes your service more attractive than others. For instance we'll pretend you have helped performers at a famous theater, their ballet were suffering from stress and physical strain. You can now say have experience with prestigious performers and artists. This is something other reflexologists may not be able to claim.
We'll try another example. Let's say you are in retail and sell photographic equipment. You've made a point of recruiting staff who are amateur photographers with a keen interest. Now you can advertise your store is run for photographers BY photographers.
It's easy to overlook the obvious
The examples above demonstrates things you might take for granted about yourself or your business. These could be very important to your advertising and marketing.
Try making a list of the things you are interested in even if they have little to do with your business itself. Then a list of things you feel safe to assume about your customers, for instance, for a camera shop it's reasonable to assume they are interested in taking pictures. When do people take pictures? Birthdays, Holidays, Special Events etc. Now try to connect one list with the other and you'll find either easy direct connections or at least a train of thought that may help you to pin-point something unique you can use to promote your business.