What does it mean to sort beans?
Sorting means a few things:


Remove foreign objects like small stones, other seeds, twigs.... that may have accidentally been packed with the beans.
Remove damaged (think insect damage, for example) or otherwise shrivelled, infected or moldy beans and loose skins. Hint: Hollow beans and skins float up.
Double-check for bug infections. Sometimes there are little hitchhikers, even if you have impeccable hygiene at home.


It boils down to making sure you have nothing in the pot that doesn't belong in your meal.