Something that's lucid is clear and understandable. Lucid writing is important in journalism, so that readers easily get the point of the article they're reading.
When what you write or say is lucid, it's straightforward and its meaning is crystal clear. You can also use the adjective lucid to describe your mind or thoughts when you're thinking in a rational, sensible way: "I was worried about my grandmother's confusion yesterday, but she seems really lucid today." Another meaning is "translucent," or "letting light shine through" — which makes sense since lucid comes from the Latin lucidus, "light or clear," with its root of lux, "light."