Like an hour glass, it is said, you simply measure the parent and the daughter elements and you can calculate the age.
These geologic events can cause elements to be gained and lost to the rock.
It’s like cracking the hourglass and having some of the sand leak out, or other sand leak in.
Radioactive decay is where the parent atoms change as a result of radioactive decay into daughter atoms, like the individual grains of sand falling from the top to the bottom of the glass.
The hourglass depends on the sand falling at a regular rate.
Some small pieces of other rock, or even some foreign minerals, may have been carried along by the magma and existed long before the rock crystallized.