Your phone's GPS may not always be able to pinpoint your exact location. If you're indoors, your phone may have trouble establishing a good connection to the satellite(s) responsible for letting it find your location.
The pin displayed in your Uber app tells your driver where to pick you up. By moving the map on the Uber app you can recenter the pin so that it accurately reflects your location. On the latest version of the iPhone app, you can also click ‘Pickup Location’ and manually enter in your address.