Arabs were travelling by camels in the desert in cravans and tribes and when a cravan wanted to have a rest , some tribes invite them to rest in their place in the travel
so when they meet those passengers, they greet them warmly by saying: (halaltum ahlan wa nazaltum sahlan )حللتم أهلا و نزلتم سهلا
literally means : you’ve been hosted among your family and you’ve been rested in a vally
vally refers to the most comfortable place in the desert that has water and palm trees and some plants like an oasis
so, after many years, Arabs used to say this greeting in brief: أهلا و سهلا
when Arabs meet visitors specially the new visitors to show how they are friendly and happy by their coming.