Borer means to separate and refine and is one of the thirty-nine forbidden Av Melochos of Shabbos (areas of creative work that are prohibited on Shabbos). This includes the prohibition to separate something unwanted from something wanted; or to separate food with a vessel used for separating such as a strainer or a sieve; or to separate food to eat after some time. This is because in the construction of the Mishkon, Borer was performed as part of the process of manufacturing dyes. After the dye plants were threshed, any impurities that could not be removed by winnowing, such as rocks and pebbles were hand-selected out of the mixture.
Borer is forbidden: Just as Borer is forbidden with food on Shabbos so too the spiritual Avodah of Birurim which prepares the material world to be a suitable dwelling place for HaShem’s Divine presence by separating the good in this world from the bad of this world is likewise not done on Shabbos but in the week.
