If your driver arrives at the pick up point and isn't able to find you there, they'll wait 5 minutes for you. Keep in mind that you may be charged an additional fee for the waiting time if the driver has to wait for longer than five minutes. For bookings, this fee will always be charged in the event that the driver has to wait later than the time of the booking.
If you find that you can't make it to the pick up point in time, the best thing to do is call the driver and tell them where you are so they can come and collect you. Your location will become the new pick up point. Bear in mind, the price of your journey may be affected.