Hi or Low Resolution refers to the density of pixels in an image. Dots per inch (DPI) or Pixels per inch (PPI) are units of measure that communicate the density of pixels in an image.
Images used for the web will have lower resolution, with 72 DPI as the standard.
Images used for print will have higher resolution, with 300 DPI or greater as the standard.
Be careful not to create an image for print that is low-res; otherwise, it will end up pixelated and distorted; or an image for the web that is too hi-res, otherwise it will take forever to load.