The UI is the visible surface of software. Every screen, button, form, menu, colour and font choice is part of it. If UX is how the whole experience feels, UI is the concrete stuff on the screen that creates the feeling.
The two travel together ("UI/UX") but are not the same. A product can be beautiful and still confusing: good UI, bad UX. It can also be ugly and perfectly usable. The best products are neither.
Why you keep hearing it
Because interfaces are having a moment. AI tools now generate working UI from a text description in seconds, which has flooded the world with decent-looking screens. Designers debate what this means for their craft. Builders show off AI-generated UIs daily. And when a chat-based product adds buttons and panels, people say it "finally got a real UI".
What it means for you
The UI is where your brand physically meets the user, pixel by pixel. Fonts, colours, spacing and tone in an interface are brand decisions, not just design decisions. If AI-generated UI is becoming cheap and everywhere, then the distinctive part (the taste, the brand, the voice in the details) is precisely what stops your product from looking like everyone else's.