The Tully-Fisher relation (TFR) is a relation between the luminosity of a spiral galaxy and the width of its emission lines. The luminosity is related to the galaxy's stellar mass and the emission spectral line width is a measure of its rotation velocity. Thus Doppler shift measurements of the rotation yield an estimate of its luminosity, making it a standard candle, whose distance can be estimated.
The baryonic Tully-Fisher relation (BTFR) is a variant, specifically using K band measurements, which more closely relate to the stellar mass. The baryonic mass is related to the velocity to the power of 3.5 to 4.