Image resolution refers to how much detail is portrayed in an image. The usual instances photographers will need to think about image resolution are when choosing a camera (megapixels,) and when exporting images for print or the web.
A higher resolution equals more detail. For example, a 36-megapixel camera can provide substantially more detail than a 12-megapixel camera. One megapixel is one million pixels, so a 36 each image produced using a 36-megapixel camera is made up of 36 million pixels, whereas the 12-megapixel camera’s images only contain 12 million pixels of detail data.
When exporting images, a resolution of pixel ‘Width x Height’ is chosen by the user. A low-resolution image is fine for small image reproduction, but if you take that low-resolution image and try to stretch it across a screen it will become pixelated in appearance as its lack of detail is revealed.