An unbelievable experience or situation is one that's hard to believe because it's so surprising or amazing. It could also mean something that doesn't make sense logically.
When people talk about something being unbelievable, they often mean it's so unusual or incredible that you wouldn't expect it to happen. For example, if someone tells you a story about winning the lottery twice in one week, you might think their luck is unbelievable. You're not necessarily saying they're lying, just that what they experienced seems too good (or bad) to be true.
Not to be believed.
