A render pipeline is an assembly line run dozens of times per second: update the world, draw geometry through vertex and fragment shaders, composite passes, apply post-processing, present to the display. Every real-time visual system is some arrangement of this line.
The frame budget rules it: 60 frames per second allows 16.7 milliseconds for everything, and each stage spends from the same account. Optimization is deciding which stage deserves the milliseconds the look actually needs.
Understanding the pipeline explains behavior that otherwise mystifies: why resolution changes cost what they cost, why one heavy effect halves the frame rate, why the last pass in the chain owns the final word on color.