Much like an heirloom possession - being something of value that has been passed down from generation to generation - heirloom seeds have a value a represent our heritage of diverse plants whether in relation to their yield, flavour, resistance to disease or something else entirely. They have been saved year by year, generation by generation for a reason.
An heirloom seed is an old variety (by definition pre-dating 1960’s) that is open pollinated, by either wind, insect or animal (humans included). Seeds collected will then produce a (nearly) exact copy of their parent plant, so by saving seeds from last season’s best tomato plant, we can enjoy a whole garden of them this year. In this way, we can slowly select for qualities that we like and evolve our gardens over time.