A publishing imprint is a trade name under which a work is published. Sometimes the imprint will be identified by a logo.
For example, my imprint is Shadenet Publishing (also the trading name for my editing business). It has a simple logo (see below, enlarged):
If you plan to self-publish a novel with a UK ISBN, you need to register with the Nielsen agency, and in doing so, you will create an imprint for yourself. It need only be your name, but it will become associated with you as a publisher, so you may want to think of something more creative.