Your unique selling points, also known as unique selling proposition ("USP") is what makes your business and its products/services different, it also defines what your brand stands for. Your USP is what's unique about your business, what you offer, who you do it for and why someone should buy from you.
A USP is not a tagline/slogan and a tagline/slogan is not a USP. A tagline is a short sentence or phrase that's often part of, or next to a logo.
This prompt will help you to start defining it:
I'm a [job title]
for [ideal client description]
who want [goal]
I help them [results]
so they can [outcome].
An example to apply to my own graphic design business would be:
I'm a Graphic Designer
for small businesses
who want to create a sophisticated and distinctive brand
but are on a start-up budget.
I help them to develop the look and feel of their business
so they can stand out in their market place.
I really hope that helps!