Your ticket is valid from 15 minutes before the bus you booked is due to depart. The ticket QR code will appear in the app 15 minutes before the bus is due to depart.
The ticket remains valid for 30 minutes after the bus you originally booked is due to arrive. This means that you can board up to 30 minutes after the bus you originally booked. In other words you can take a slightly earlier or later bus. Tickets for bus services operated by HTM or RET are valid for 2 hours.