David Luiz had a very up-and-down season with Chelsea, regularly dropping in and out of the lineup. He was very solid as a defensive midfielder, but wasn't first choice at that spot due to Nemanja Matic, nor was he preferred to Gary Cahill or John Terry at the end of the season.
The presence of those three players made him expendable for Chelsea, and it looks like they'll turn a tidy profit. They've agreed to sell him to Paris Saint-Germain.
The fee PSG will pay has varied wildly in reports, but it's almost certain to be more than the €42 million that the French club paid for Thiago Silva, which was a record for a defender.
Silva is still in the prime of his career, while Marquinhos had a very solid first season for PSG, so it's not entirely clear why they'd like to spend such a large sum of money for Luiz. They're also facing financial fair play restrictions, so they'll be unable to make other marquee signings unless they make huge sales as well.
It's a very curious move by PSG, and a coup for Chelsea. They've sold a player they weren't going to start next season for a record fee for a defender, to a team that supposedly shouldn't be spending lots of money.