Description. As writers, this is something we all need to practice because it is such an important tool. Every object has a size, shape and color (or lack thereof), and plenty of them also have smells or odors, flavors, textures, they make sounds, they move in certain ways, and so on. Look at any object and consider that you could write a paragraph just describing the details of that object. Then, you could write another paragraph writing about why those details are important, or the context behind a particular aspect. However, part of the art of writing is knowing when all this information is worth writing about in the first place.
The most important use of description is to draw the focus of the reader in on one particular part of the world. It is the written version of zooming in, and giving the reader a more intimate experience. This also contributes to the mood and feeling of a scene, making the world very real, very tactile. Describing, say, a crowded kitchen fills in the image but also creates the mood of a chaotic, non-stop, whirlwind of activity going on within this one room. In that regard, it's very necessary. However, in other cases, it's not even relevant.
One question we can ask ourselves when deciding how much attention to offer in describing a person, item, or scene is how much it adds to the overall experience. In our crowded kitchen scene above, does it help to describe all the different foods being hurried around, or do we get the same effect if we focus on the chef, scrambling between the stove and the prep counter, dodging his assistants and impatient waiters, trying to make up for the time he lost because someone spilled broth all over the chicken he had been preparing? If we describe the entire kitchen, the reader sees the chaos. However, if we describe that one chef struggling through everything rather than the whole kitchen, the tighter focus means the reader becomes part of the chaos.
The most important question we need to ask ourselves with description is, "What do I want to achieve with this?" Sometimes it serves the reader to create a mood, sometimes it gives the reader a point to focus on, and sometimes, good description creates dimension to an otherwise flat scene.
And sometimes, the car is blue.