Shiny in a not very bright way, like a little bit shiny.
If something is semilustrous, it is a little shiny, but not very much.
Examples:
The rock is semilustrous, it shines a little bit in the sun.
Her hair is semilustrous, it's not very shiny, but a little bit.
The semilustrous coin caught my eye.
The cat's fur is semilustrous, it's not very shiny, but a little bit.
The semilustrous stars twinkled in the night sky.
The semilustrous paint made the room look a little brighter.