It is an expression that people say to others who are leaving the house, etc, and literally just means "please go and come back." People in other cultures may just say "see you later" instead.

That expresion is very particular the way to say when someone is leaving home. Saying "いってらっしゃい" is, personaly, with praying he\she will return home safe, and also including the feeling of good luck (on your studies, work, etc..). In other countries, they say just "bye" or such.