Your screen resolution is expressed in pixels. One pixel is just one tiny dot on your screen that has a color. If you put a bunch of pixels together, you can, for example, make a drawing. All screens, including the screen of your phone and computer, consists of thousands to millions of pixels. A blue rectangle on your screen is just a collection of blue pixels. On most modern screens you can't see the individual pixels/dots, but if you would zoom in, you would definitely see them.
Your screen resolution tells about the number of pixels your current screen is using. The more pixels it uses, the more items can fit on it. Let's say you have a resolution of 1280x720. This means your screen has 1280 pixels horizontally and 720 vertically. It's like a raster. So in the case of 1280x720, your screen consists of 1280*720 = 921600 pixels.
There are various situations where people change their screen resolutions: