UI is short and techy for User Interface, It can refer to what a user of your product/app/service see’s in their interaction and use. Did they like the layout? was there glitches and bugs? did the colours or theme suit? that stuff.
UI can be quite expansive, it covers everything like fonts through to the size of a logo on a screen. It can be strenuous to be a UI tester and you really do need talent to be a UI designer. UI designers need a greater understanding of the company and the vision, to ensure that the UI design is suiting in its aesthetics.
When making something a pleasure to use, UI designers will need feedback with the UX team (User experience). If your app is not pleasing to use, spacing is wrong and users are finding the layout off-putting your core business or service will suffer.