What is anti-aliasing?
Antialiasing is a technique used in computer graphics that reduce the visual defects that occur when high-resolution images are presented in a lower resolution. It occurs because of the output device, the monitor or printer, doesn't have a high enough resolution to represent a smooth line.
This is also an inherent problem on a computer. The pixels that make up the screen of the monitor are all shaped in rectangles or squares. Because lighting up only half of one of these square pixels is not possible, the result is roughness or jagged line.
The jagged line effect can be minimized by increasing the resolution of the monitor, making the pixels small enough that the human eye cannot distinguish them individually.
But, this is not a good solution. Because images are displayed based on their resolution and a single image pixel may take up many monitor pixels, making it impossible for a higher resolution monitor to mask the rough edges.