It refers to the clarity of an image on screen. Screen resolution suggests the number of dots or pixels on the entire computer screen. For example, when you say a 640 x 480 screen resolution then all you means is individual 640 dots on each 480 lines i.e. 307K pixels.
Use xdpyinfo command to find out current screen resolution:
$ xdpyinfo | grep 'dimensions:'
dimensions: 800x600 pixels (283x212 millimeters)
You can also use xrandr command:
$ xrandr | grep '*'
*0 1024 x 768 ( 283mm x 212mm ) *61
You can also use Desktop tools to find out current desktop screen resolution:
(A) Gnome Desktop
Click Gnome Desktop menu > Preferences > Screen resolution
- Click on K desktop Icon > Select Control Center
- Select Peripherals (under Index tab) > Select Display
- It will display Screen resolution or size
See Screen resolution at wikipedia.