I am not an Islamic scholar, so if you want to find out more detail about this phrase I’m sure someone who understands Islam would be pleased to explain it to you further.
The phrase Behind the Sun does not appear to be significant in the teaching of Islam.
However I know that the sun is extremely important in the day to day life of every Muslim.
In the past they relied on it’s position on the horizon to calculate the correct time to pray and to fast. (Nowadays of course most Muslims, whether they live in towns or in the country, rely on pre-determined prayer times worked out by a computer)
Some people believe that if a man sees himself walking ‘behind the sun’ in a dream then this signifies capture and imprisonment.
I think the best explanation I can find says that if a person is ‘put behind the sun’ they are outcast and forgotten.
Hope this is useful!