Thank you!
Cel shading is the stylised form of rendering developed in response to the limitations of old cel-based animation. Cels are transparent sheets that artists apply ink and/or paint to, then layer and photograph to create animation frames. For time and budget reasons, each cel was coloured with large volumes of flat colour with no blending between tones.
The resulting style is effective, striking, and broadly well liked; which is why modern digital 2D animation is often cel shaded and the effect is replicated intentionally in 3D despite the original limitations being completely gone. Hope this helps!