A fraction represents a point on a number line. The fraction 1/2, for instance, represents the midpoint between 0 and 1 on the number line. When fractions are in order from least to greatest, the points they represent are in order left to right on the number line.
When you are told to put fractions in order least to greatest, it means you are to identify the fractions in the order of the points they represent on the number line when those points are read left to right.
This picture shows the portion of the number line between 0 and 1 with some fractions marked on it. If we are asked to put the fractions 2/3, 3/4, and 1/6 in order least to greatest, we would do this. Find each of these on the number line. Read them in order left to right as they appear on the number line. That order is 1/6, 2/3, 3/4.
Of course, if you know how to compare the magnitudes of fractions and/or decimal numbers, you don't actually have to place them on the number line.