The shank or shaft of a bur is the base which holds the neck and head of the bur in place.
They should be of stainless steel composition (to prevent rusting) and exactly 2.35mm in diameter.
Even the smallest of variations in shank diameter can cause the bur to ‘slip’ when in use, causing strain on your drill. Cheap burs often have a shank that varies in diameter which means there is limited surface area for your drill to ‘grip’ the bur.