A fractal antenna is an antenna using a fractal, self-similar (repeating) design to maximize the effective length, or increase the perimeter (on inside sections or the outer structure), of material that can receive or transmit electromagnetic radiation within a given total surface area or volume.
[1] Fig. 1 An example of a fractal antenna: a space-filling curve called a "Minkowski Island"[1] or "Minkowski fractal"[2]
Multiband performance: Since the antenna features geometry repeated at different scales, each scale resonates at a different frequency. This allows a single physical structure to operate efficiently across multiple frequency bands (e.g., Cellular, Wi-Fi, and GPS).
Miniaturization: The complex, space-filling nature of fractals maximizes the effective electrical length of the antenna within a smaller total volume or surface area.
Wideband capability: Some geometries create continuous frequency coverage rather than discrete bands, providing ultra-wideband (UWB) operation.
[1] Wikipedia