Modern day computer has dedicated Graphics Processing Unit (GPU) to produce images for the display, with its own graphics memory (or Video RAM or VRAM).
Pixels and Frame
All modern displays are raster-based. A raster is a 2D rectangular grid of pixels (or picture elements).
Frame Buffer and Refresh Rate
Double Buffering and VSync
While the display is reading from the frame buffer to display the current frame, we might be updating its contents for the next frame (not necessarily in raster-scan manner). This would result in the so-called tearing, in which the screen shows parts of the old frame and parts of the new frame.
This could be resolved by using so-called double buffering. Instead of using a single frame buffer, modern GPU uses two of them: a front buffer and a back buffer. The display reads from the front buffer, while we can write the next frame to the back buffer. When we finish, we signal to GPU to swap the front and back buffer (known as buffer swap or page flip).