When discussing the brightness of stars we typically refer to apparent and absolute magnitude. Apparent magnitude of a star how bright we observe a star from Earth compared to the star Vega. Stars bright than Vega have a lower magnitude, while stars dimmer have higher magnitude than Vega. Stars that are further away will have a lower apparent magnitude. Therefore, we can use this relationship to calculate how far away a star is, if we know its absolute magnitude. The absolute magnitude is how bright a star would be from some standard distance, which is 10 parsecs. Knowing the spectral type of the star can help identify its absolute magnitude.
Type 1a supernovas occur when white dwarf stars pull material of a nearby red giant star. Once the white dwarf hits the upper mass limit of 1.4 solar masses it approaches the ignition temperature for carbon. This causes a runway thermonuclear reaction which is observed as a supernova. As these types of supernovas occur at the same mass each time they have the same absolute magnitude. Therefore, they can be used to calculate distances in the universe in the same way as stars.