Well if we started off with say, to simplify things,100 units of a parent isotope when this nature started, and we no, due to decay, only had say, 99 units, that would mean one unit decayed away in the last 4300 or whatever time since this nature existed. Or, if there was say, 100 units of what is now daughter isotope at the start of this nature, and we now had 101 units, that would mean we got one extra unit from decay.
The pattern in this example would be one more or less unit for either the parent or daughter material that came to exist since the sate change. But because of the current processes in this nature, if we looked at the 99 or 101 units, we would assume they came to exist because of decay. We would then assign an age based on the rate of decay now observed for all 99...or 101 units rather than just the one unit that actually came about from current nature decay!