In order to get defensive tackle Haloti Ngata from the Baltimore Ravens, the Detroit Lions sent them two 2015 draft picks. In exchange, the Lions got Ngata and a 2015 draft pick of their own. Via Adam Schefter, here are the details:
DT Haloti Ngata
Seventh-round pick in 2015
Fourth-round pick in 2015
Fifth-round pick in 2015
Unless the Lions end up getting a compensatory pick -- which isn't projected to happen -- they are going to have a long wait between picking in the third round and going back on the clock in the sixth round. I suppose they could always make additional moves to get an extra pick in the fourth or fifth round, but if not, that's a big gap in between selections.
In any case, the Lions clearly didn't want to play around with the possibility of Ngata being dealt elsewhere or being released and signing with another team. They had to do something to make up for the loss of Ndamukong Suh, and this move cost them a pair of mid-round picks.