A single gradient blends only two colours; a mesh gradient layers several radial-gradients at different positions and colours so they bleed into one smooth field, each fading to transparent so they melt together.
Slowly rotating the whole thing through hue-rotate gives a flowing colour shift. CSS can't do true triangulated mesh interpolation — this is an approximation by stacking radial-gradients. Tools like Figma's Mesh Gradient plugin or CSS Houdini can produce a more precise version.
When to use
Use for hero backgrounds, OG images, brand backgrounds. If placing text on top, contrast shifts because light and dark areas aren’t fixed — prepare an overlay or a fixed text colour.