Full Idea
If it were asserted that the Form of the Beautiful was itself beautiful, such a statement would require a prior understanding of the concept of beauty, so would immediately lead to an infinite regress, so the Forms can't be self-predicating.
Gist of Idea
A prior understanding of beauty is needed to assert that the Form of the Beautiful is beautiful
Source
Luke Westaway (talk [2005]), quoted by PG - Db (ideas)
A Reaction
This is a nice clear statement of the mess that Plato gets himself into if he wants the Forms to be self-predicating. Clearly the Form of the Beautiful can't be beautiful, but must be that which gives other things their beauty.