Lit up by the sun — a room, a street, a hillside with sunlight falling on it. It describes the thing the light lands on, not the light itself, and it is almost always positive: warm, bright, pleasant. Like sunless, it belongs more to writing than to everyday talk; in speech you would say "the sun was coming in" or "it's really bright in here". Estate agents love it for describing flats. You will also see the fixed phrase "sunlit uplands", meaning a happy future after a hard time — often used with a raised eyebrow now, because the promised good times so often fail to arrive.