The gravity model predicts the spatial interaction (trips, trade, migration, calls) between two places as I = (P₁·P₂) / d². Bigger places attract more; interaction falls off with the square of distance — that steep falloff is distance decay.
Top: city sizes & flow. Bottom: interaction vs distance (the distance-decay curve); the dot marks your current distance.
Try: note the interaction, then double the distance. The interaction does not halve — it drops to one quarter, because d is squared. That is the heart of distance decay.