It is the "thickness" of the cable. The thicker it is, the more current it can handle without its temperature rising.

3 x 2.5

Intended for heavier loads, such as general-purpose sockets, power tools, ovens, or refrigerators. It can safely support single-phase power supply lines.

Single-phase

The single-phase extension reel connects to a household or professional 230V AC supply. It is intended for low or medium power devices, such as lamps, computers, or tools. The operating current usually ranges between 10–16A. It is easy to use and transport, but cannot power high-power machines. It is recommended to use cables of suitable cross-section to avoid overheating.