When we measure the narrow line emissions of hydrogen at 21-cm radio wavelengths along a particular line of sight through the disk of the Milky Way Galaxy,we can tell the distances to different hydrogen clouds because:

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Clouds at different distances have different Doppler shifts because of the rotation of the Galaxy. This is known as the Galactic rotation curve. By measuring these Doppler shifts, we can determine the radial velocity of the clouds and, hence, their distance from us. This technique is called radio astronomy and is used to map the structure of the Milky Way Galaxy.

When We Measure the Narrow Line Emissions of Hydrogen at 21-Cm