Help needed with syntax in C3_W1_Assignment - Exercise 2

In exercise 2, for the compute_centroids function, what does this line do exactly? (having a bit of trouble to understand the syntax)
points = X[idx == k]

X and idx are arrays, k is an integer. idx==k checks each element in the array idx whether the element is equal to k, returning an array of booleans (True / False), for example idx==k may give us an array like np.array([True, False, True, False]) indicating the results of the checks element-wise. X takes the checking result and filters its own element with the checking results - leaving only element where the cooresponding check is True . For example, if X is np.array([4,5,6,7]), then X[idx==k] will give us np.array([4, 6]) because only the zeroth and the second checking results are True.