Wholesale odds are the "real odds" or 100% probability of an event occurring. This 100% book is displayed without any bookmaker's profit margin, often referred to as a bookmaker's "overround" built in. The book price is effectively a measure of the competitiveness of the odds prices on offer.
A "wholesale odds" index is an index of all the prices in a probabilistic market operating at 100% competitiveness and displayed without any profit margin factored for market participants.
References
Wholesale odds Wikipedia(Text) CC BY-SA