const series = d3.stack()
.keys(d3.union(this.data.map(d => d.age))) // distinct series keys, in input order
.value(([,D], key, ) => D.get(key).population) // get value for each series key and stack
(d3.index(this.data, d => d.state, d => d.age)); // group by stack then series key

I don't understand what does mean the value() function?

Please help.