This is the 248th most frequent German word.
bis
"Bis" means "until" or "to" in English.
The word 'bis' in this sentence specifies the point of time or event (your return) up to which the action (waiting) will take place.
Ich warte hier, bis du zurückkommst.
I will wait here until you return.
Here, 'bis' is used to describe the endpoint in a range of time during which an ongoing action (store being open) is happening.
Der Laden ist von 9 Uhr bis 18 Uhr geöffnet.
The store is open from 9 AM until 6 PM.
In this phrase, 'bis' is used as part of a colloquial farewell, indicating the next encounter is pending until a later moment.