The Foundation pretends to be a charity but spends as little as 10% of the donations on actual charity.
The Foundation is a conduit for foreigners to contribute to an elected US official, such as Hillary Clinton (note that direct foreign contributions to US politicians and US political action committees are illegal).
I'm not claiming the Clintons are directly stealing money from their Foundation, although they are so sleazy that would not surprise me -- the Clintons tend to be too clever for that.
The Clintons ask contributors to hire Bill, for hundreds of thousands of dollars, to travel somewhere and make a short speech about his "wonderful" Foundation.
Or they pay Hillary for a speech (when she was not a Senator or Secretary of State).
The Clinton Foundation pays for the travel, hotel and food required for the trip to make a speech -- so the speaker essentially gets a first class vacation in return for a short speech, and shaking hands.