Reference multiple rows of one table from another
Setting the column "needed_for_bundle" as a Foreign Key that references the Primary Key of the table Bundle would mean that there's a one-to-many relationship between the two tables. Each product has to belong to one bundle, and one bundle can be composed of one or * product(s).

So first, needed_for_bundle should be the same type as the Primary key of Bundle table which is INT(6).

After changing that you can apply this code, for the foreign key constraint + index:

ALTER TABLE `Product` 
ADD FOREIGN KEY (`needed_for_bundle`) 
    REFERENCES `Bundle` (`id`) 
    ON UPDATE CASCADE 
    ON DELETE CASCADE;
CREATE INDEX `fk-id_bundle-Product-Bundle` 
    ON `Product` (`needed_for_bundle`) USING BTREE;