Nean Der Thal's answer is correct regarding the meaning of the +1
(it means the flight lands the next day.)
More generally, the number displayed there is the difference between the date of the arrival and the date of the departure in the respective local times of those locations. This can range anything from -1
(landing the day before you took off) to +2
(landing 2 days later than you took off.) Theoretically, more than +2
is possible, but only in a really slow aircraft that can stay airborne for a very long time (such as a blimp.) +2
is the most you'll see on a normal jet airliner, though (thankfully.)
Which flights are likely to experience each situation varies dramatically by region. While Nean Der Thal is correct that +1
is most likely to happen on East-bound flights in most parts of the world, the opposite is true for trans-Pacific flights which cross the International Date Line. On trans-Pac flights, almost all West-bound flights will be at least +1
(and often +2
for overnight flights that leave before midnight local time,) while East-bound flights will almost never be +1
(and can occasionally be -1
for flights that depart in the early morning hours local time.) Also, +1 is very common on overnight North-bound or South-bound flights in any region.