The visible part of the design. What the visitor sees, touches and bases their judgement on.
The look of the screens themselves: which typeface, which colours, how much space between elements, what a button looks like. Where UX is about whether something works, UI is about how it looks while it works.
Light grey text on white looks calm and is unreadable for some of your visitors. The accessibility guidelines set minimum contrast ratios, and we meet them — not least because it simply reads better on a phone in the sun.
A design with short, made-up copy falls apart as soon as the actual heading takes up two lines. We design with the longest version that could occur, not the prettiest.
A question or project on this topic?