From the javadocs, it means:
(Optional) The foreign key columns of the join table which reference the primary table of the entity that does not own the association
In layman’s terms, it is the column of Category that will be used as a part of the JoinTable relationship between the current entity and Category.
If you don’t specify joinColumns and inverseJoinColumns on the @JoinTable annotation, the persistence provider assumes a primary key to primary key join relationship and still store the equivalent ID columns for two related entities in the table by default.