Those numbers — 1.0, 1.5, 2.5 and so on — are the cross-sectional area of the conductor in square millimetres (sq mm), i.e. how much metal is inside the insulation. The larger the area, the more current the wire can carry safely and the lower its resistance and voltage drop over a run.
As a rough guide for Indian homes (always confirm with a licensed electrician against the actual load and IS 694 / wiring regulations): 1.0–1.5 sq mm is typically used for lighting circuits; 2.5 sq mm for general power sockets and ring circuits; and 4–6 sq mm for heavy appliances such as air-conditioners, geysers and ovens. Bigger sub-mains and the incoming supply use larger sizes again.
Choosing the size is not just about the appliance rating. It also depends on the current-carrying capacity (ampacity) for that installation method, the length of the run (longer runs may need a size up to limit voltage drop), grouping with other cables, and ambient temperature. This is why a rule of thumb is a starting point, not a substitute for a proper calculation.
Under-sizing is dangerous: a conductor that is too thin for its load runs hot, ages its insulation prematurely and becomes a fire risk. When in doubt, size up and have the design checked. Our guide on how to choose the right cable walks through the factors.
APAR house wires are available across the common sizes in FR-PVC and low-smoke HR-FR-LSH grades — see Shakti FR-PVC, Anushakti HR-FR-LSH or the full house wires and cables range.