Glass refracts different wavelengths by different amounts — the same dispersion that makes a prism produce a rainbow. When a lens cannot bring red, green, and blue to a common focus, edges pick up colored halos, typically purple or green, and most visibly along high-contrast boundaries near the frame edges.
There are two kinds. Longitudinal aberration means different colors focus at different distances, producing fringing in front of and behind the plane of focus even at the centre; it worsens at wide apertures. Lateral aberration means colors land at different positions across the frame, growing toward the corners.
Lens designers fight it with low-dispersion and fluorite elements, which is much of what you pay for in expensive glass. Lateral aberration corrects almost perfectly in software using lens profiles; longitudinal is far harder to remove cleanly. Stopping down a stop or two reduces both noticeably.