The number of pixels in an image. Resolution is expressed in terms of the width and height of the image in pixels (for example, 640 x 360 pixels). Higher-resolution images contain more detail but also create larger files that take longer to download.
Your electronic devices (Mac, iPhone, iPad, iPod, and so on) also have screen resolution. Ideally, you should match the image resolution of your media to the resolution of your playback device.