User experience is, at the core, what a human feels while engaging with a company's product or service.
Good user experience engages users with multiple disciplines* of design to allow a clear and pleasant process when using a product/service. Good UX pleasantly surprises people and creates an emotional connection between them and the product.
Bad user experience confuses users and creates road blocks where none need to exist. It leaves a bad taste in a user's mouth that effects all other encounters they have with that product again.
* What I mean by "multiple disciplines" is that user experience isn't just creating wireframes. The whole of the experience a user encounters with a product includes the visual design, interactions, branding, as well as the information design.