Wash your windshield! Make sure the wipers are working! Good visibility is paramount to safe driving, but also for appreciating a photo. That’s why focus is important.
Technically speaking, focus is achieved when the light rays emanating from each point of the subject converge onto one point on the camera’s sensor. On the other hand, if the light rays do not converge onto one point on the sensor, but converge above or below the surface of the sensor, the image will be out of focus. Practically speaking, in a magnified, out-of-focus image, you will see blurry circles rather than the points characteristic of a well-focused image. The ideal focus in each situation depends on factors such as the distance of the subject from the camera lens, as well as the lens itself, and its minimum focal distance. Also, when shooting at low speeds, the very action of taking the photo (with the inevitable camera shake) can cause a blurry image. That’s why it’s important to use a tripod when working below certain shutter speeds.
Here is an example of focus problems.
Example. This image was shot in a temple with very low lighting. Probably, it was shot without tripod at a low shutter speed, which is why it isn’t sharply focused. In the detail on the right, you can see that the statue’s face is blurry.