These cylindrical plastic cleats are a locking system. Indeed, this plastic part is made to hold the corner of a cabinet, a piece of furniture.
Their standard availability in white, black, brown (RAL 8011) of PA or PE material according to your needs.
It is a cylindrical cleat either half-washer or washer, which is used to support the end of a cleat or a bar that cannot be stopped with nails.
They are mainly used in carpentry.