In fact yalla is an arabic term and it consists of two words:
1. Ya which is like a preposition usually used to call someone else.
2. Allah, which means God.
Actually, yalla has too many usages like acceptannce, confirmation, urgency and many other meanings.
Yalla imshi : let's go
Yalla jayi : I am coming
Yalla wlow : Hurry up
and so on...