Image resolution usually means the pixel dimensions of a photo, such as 6000×4000 pixels. Multiply those numbers and you get the total pixel count (24 megapixels in this example).

For printing, those pixels are spread over a physical size. The more pixels you place into each inch of paper, the more detail the print can show. A common rule of thumb is around 300 dpi/ppi for good photo prints. So a 6000×4000 image can make roughly a 20×13.3 inch print at 300 ppi.

For screens, the image is also shown with pixels, but the important factor is mainly whether the image has enough pixels for the display size. Screens and video generally use lower pixel density than print, so an image that looks fine on a monitor may not support a very large, detailed print.

“High resolution” is relative to the use. A file may be high resolution for web or video, but not especially high for a large print. In practice, “high” just means enough pixel detail for the intended output size and viewing distance.