You have learned a trick that makes your spellcasting more flexible.
Benefit: When you prepare your spells each day, you can choose to keep a number of spell slots equal to your Intelligence modifier (minimum 1) flexibly prepared. Instead of preparing a single spell within each of these slots, you can partially prepare two spells. Anytime after you prepare these flexible slots, you can spend a full-round action to finalize one slot, choosing one of the two spells to be finished and fully prepared in that slot.
Pathfinder Roleplaying Game Advanced Class Guide © 2014, Paizo Inc.; Authors: Dennis Baker, Ross Byers, Jesse Benner, Savannah Broadway, Jason Bulmahn, Jim Groves, Tim Hitchcock, Tracy Hurley, Jonathan H. Keith, Will McCardell, Dale C. McCoy, Jr., Tom Phillips, Stephen Radney-MacFarland, Thomas M. Reid, Sean K Reynolds, Tork Shaw, Owen K.C. Stephens, and Russ Taylor.