The stars and gas in the disks of spiral galaxies orbit about the center of the galaxy as evidenced by their rotation curves. Because of the motion of the gas and stars, the emission from the orbiting stars and gas suffers Doppler Shifts. Because the size of the Doppler Shifts,
, tells us how fast the source moves, we can infer information about the mass of the galaxy. This fact is taken account of by the Tully-Fisher Effect

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The Tully-Fisher Effect must be calibrated. This is done in a series of steps: parallax ====> distance to nearby stars ====> distances to nearby clusters of stars (calibrates bright stars and Cepheids in clusters) ====> distances to nearby galaxies (calibrates Tully-Fisher Effect). This bootstrapping leads to what the text refers to as the Distance Ladder. This important effort allows us to calibrate the Hubble Law, by giving us the distance to galaxies with measured redshits, z.