This is the 46th most frequent German word.
ab
"Ab" in German means "off," "from," or "starting at," depending on context.
In this sentence, 'ab' is part of the separable verb 'abfahren', which means 'to depart'.
Der Zug fährt morgen ab.
The train departs tomorrow.
Here, 'ab' is used as a preposition meaning 'starting at' or 'from' to indicate a starting point for prices.
Ich kaufe die Tickets ab 10 Euro.
I buy the tickets starting at 10 euros.
In this instance, 'ab' is part of the separable verb 'ablaufen', which means 'to drain' or 'to expire'.