When it comes to responsive images, understanding image resolution and density is crucial. Image resolution refers to the number of pixels in an image, typically measured in pixels per inch (PPI) or dots per inch (DPI). On the other hand, image density refers to the number of pixels that fit into a given physical space on a display, commonly referred to as pixel density or dots per pixel (DPP).

The relevance of image resolution and density lies in the fact that different devices and screens have varying pixel densities. For example, a high-resolution device like a modern smartphone may have a higher pixel density compared to a traditional computer screen. This means that the same image displayed on these two devices can appear differently in terms of sharpness and clarity.

To ensure that images maintain their quality on different devices, it is important to provide multiple versions of the same image, each optimized for a specific device resolution. This is where pixel density descriptors come into play. By using descriptors like 'x', which represents the device's pixel density, or 'w', which represents the device's width, we can easily provide different image versions to meet the diverse needs of various devices.