From the UEFA web site...
The winners of these 35 ties [third qualifying round] are joined in the play-off round by another 41 clubs; the 15 losing teams from the UEFA Champions League third qualifying round, the domestic cup winners from the associations ranked 1 to 17, the third-placed league sides from the associations ranked 7 to 9, the fourth-placed team from the associations ranked 4 to 6 and the fifth-placed teams from the associations ranked 1 to 3.
The 38 victorious sides from these two legs qualify for the UEFA Europa League group stage, and will be joined there by the ten losing sides from the UEFA Champions League play-off round.