Sheen is simply how much light the dried paint reflects. Flat reflects almost none, which hides every roller mark and drywall imperfection but scuffs easily and is hard to wipe. Semi-gloss reflects a lot, cleans beautifully, and shows every bump in the wall. Everything else sits in between.
For most living rooms, bedrooms, and hallways we recommend a modern matte or eggshell: it hides texture and patchwork, and today's premium lines are washable enough for real life. Kitchens, bathrooms, laundry rooms, and kids' rooms do better in satin, which handles moisture and scrubbing without looking shiny.
Trim, doors, and cabinets are where semi-gloss (or a dedicated enamel) earns its keep. The harder, slightly reflective surface resists fingerprints and chips, and the contrast against a matte wall is what makes trim look crisp and intentional.
Ceilings almost always want flat, and it's worth using a true ceiling paint rather than leftover wall paint — it's formulated to spatter less and hide the roller lap marks that show up when light rakes across a ceiling in the afternoon. If you're unsure, ask us for a sample board; seeing sheens side by side on your own wall settles the question fast.