This reaction is known as Wittig Reaction. A powerful reaction for the synthesis of Alkene. In question the starting materials are aldehyde and a Phosphorous Ylide. Ylide when reacted with aldehyde produces a four membered ring which on rearrangement gives Alkene and triphenylphosphine oxide. Phosphorous having great affinity toward the oxygen is the driving force of this rearrangement. The reaction along with product (highlighted blue) is as follow,