In physical optics, chromatic aberration is failure: glass bends wavelengths unequally and edges fringe with color. Digital imagery adopted the flaw as vocabulary, offsetting red and blue channels to suggest a stressed lens, a broken signal, a hit landing.
What causes it in a lens
A lens bends light by refraction, and refraction depends on wavelength. Blue bends more than red, so a simple lens focuses each color at a slightly different distance (axial aberration) and magnifies each color slightly differently toward the edges (lateral aberration). The result is the purple-green fringing photographers know from high-contrast edges, strongest in cheap glass, wide apertures, and frame corners. Camera makers fight it by pairing glass elements of different dispersion, and eyeglass wearers meet the same physics as color edges at the periphery of strong prescriptions.
The digital effect
Games and visuals simulate the flaw on purpose: render the frame, then sample the red, green, and blue channels at slightly different offsets or scales in post-processing. A few pixels of separation reads as analog stress, unreality, or force. It costs almost nothing on a GPU, which partly explains its overuse.
Using it well
As a reactive effect it works in bursts: a transient splits the channels for a few frames and the frame itself seems struck. Held constantly it reads as cheap music-video filter, so the good version is dosage plus falloff, strong at edges, calm at center. Its power comes from photographic memory: audiences have seen lenses fail this way, so the effect smuggles physicality into synthetic images. That memory depletes with exposure. A frame that fringes on every kick teaches the audience to stop feeling it by the second chorus.