Decor is a measurement of how pleasant the environment is to Duplicants. Every Duplicant has a decor expectation value that determines how well they will tolerate the tile space they are currently in. If a decor meter goes down below their expectation level, the stress level of that Duplicant will increase. Decor caps at +100. The Duplicant decor expectation value caps at 75.
Items in the game will either increase or decrease decor levels. Items like a Masterpiece Painting can increase decor by +20, where a pile of Compost will decrease it by the same amount. 'Ugly' items like wires, ladders, and storage will lower levels. Every item will have a value and a radius of how many tiles effect covers. Other decor items within the radius will affect the total decor sum for that tile. Try to space out the negative items and place positive items like plants and pictures between them to help counteract the negative levels.
There are other factors that can raise or lower a decor level, like what the decor is made from. Some materials are considered more pleasing than others. Also, clutter like dropped resources or wet floors can affect decor levels. To see what the current decor levels are in a given area, players can use the Decor Overlay (F8). Green represents higher decor and red represents areas with negative decor. Players can mouse over an area to see the exact sums.
When creating decor, it's a good idea to assign Duplicants who have a higher creativity level to the task. These will have a much better chance of creating higher quality paintings and sculptures. Allowing them to use Granite will help increase the levels as well.