1. Pixel and Resolution
Image Minimum Unit : Pixel
Pixel which is the minimum unit of the image is a word that is combination of a picture and element. If you magnify the picture, you can see a small rectangle with only one color called 'Pixel'. These pixels compose an image.
High-Resolution means pixels per unit area is high, low-Resolution means the opposite. Resolution is usually expressed in PPI(Pixel Per Inch) and DPI(Dots Per Inch).
▲Each Pixel has own color unit, they compose an image.
Checking the resolution difference
Normally 72ppi resolution is enough for monitor output, 300ppi or more for a printable image.300ppi is an image consisting of total 90,000 pixels, width 300pixels and height 300pixels in 1inch rectangle.72ppi image is total 5,184 pixels, 72pixels for width and height, there is considerable resolution difference.
▲ 400% Maginified 72ppi image. ▲ 300ppi image
Adjusting the Resolution in Photoshop
Menu → [Image] → [Image Size] ( Alt + Ctrl + I)
As you increase Resolution, monitor image size will change as well.
When you increase resolution of images, image will be bigger but no difference on image quality. In general, the reason for adjusting the resolution is to reduce the file size by reducing high resolution images.