The 1:1 rule is a simple way to judge a safe distance to uninvolved people in the A2 and A3 subcategories: the horizontal distance must be at least equal to the drone's flight height. If you are flying at 60 m, keep at least 60 m from people. In A2 the distance may never fall below 30 m — that floor applies even at low heights.
In A3 the principle works alongside the so-called 2-second flight distance — a buffer equal to the distance the drone would cover in 2 seconds at its maximum speed. Importantly, the 1:1 rule is only about distance and height; it has nothing to do with the drone's weight.