Money flipping is a type of scam where an unscrupulous operator will offer to increase an amount of money that you send them through so-called quirks in the money system that only they can take advantage of. The idea is that you send them an amount of money and they return it to you with a profit (that is usually so high that it is immediately suspicious).
A Western Union money flip means that the scammer asks you to deposit the money for the scam through a Western Union office to a specified account. Of course once these con artists have received your money you never hear from them again and your money is effectively lost. These types of scams are usually perpetrated through social media channels. Remember that if it sounds too good to be true then it most definitely is in the majority of cases.