UX is the whole experience of using something. Not how it looks (that leans more toward UI), but how it works for a human trying to get something done. Can you find what you came for? How many steps does checkout take? Does the app do what you expected when you tapped that button?
Good UX is mostly invisible. You only notice it when it is missing, which is why the discipline is easy to undervalue and expensive to ignore.
Why you keep hearing it
Because "the UX" has become everyday shorthand for "how usable this thing is", far beyond design teams. And because AI has raised the stakes twice over: AI tools generate interfaces quickly (making decent UX cheaper to produce), and AI products introduce brand-new UX questions, like how a chat assistant should behave when it is unsure.
What it means for you
Marketers live closer to UX than most job descriptions admit. The landing page journey, the signup flow, the unsubscribe experience: all UX, all shaping how people feel about the brand. When you hear "the UX is bad", translate it as "people are struggling to do the thing", and treat it as seriously as a message problem.