concatMap 

concatMap, as well as other **Map operators, will substitute value on the source stream with a stream of values, returned by inner function. When source stream emits, concatMap will call inner function to get inner stream that it will switch to. Once switched to inner stream, concatMap will wait for it to complete, before getting a new inner streams on following source emissions:

const { rxObserver, palette } = require('api/v0.3');
const { from, timer, pipe } = require('rxjs');
const { zip, take, map, concatMap, delayWhen } = require('rxjs/operators');


// our source$ will emit values at 5ms, 10ms, 20ms
const source$ = fromDelayed([ 5, 10, 20 ]).pipe(
    zip(from(palette), Marble) // colorize each item
  );

const concatMap$ = source$.pipe(
    concatMap(x => timer(0, 3).pipe(
        take(3),
        colorize(x.color))  // colorize as source$ value
      )
  );

// visualization
source$.subscribe(rxObserver('source$'));
concatMap$.subscribe(rxObserver('concatMap( timer(0, 3).take(3) )'));


// helpers
function colorize(color) {
  return pipe(
    map(y => Marble(y, color))
  );
}

// creates a colored Marble
function Marble(value,color) {
  return {
    valueOf: ()=>value
    , color
  };
}

// like .from, but items are delayed by their value
function fromDelayed (arr) {
  return from(arr).pipe(
      delayWhen(x=>timer(x))
    );
}

Check out this mergeMap vs exhaustMap vs switchMap vs concatMap head-to-head comparison.