Beeline knows which direction to point in by measuring the earth's magnetic field, just like a compass does.
The magnet symbol means that Beeline is measuring a magnetic disturbance.
When you see the magnet symbol appear on your device screen, it could mean one of two things:
1. If it is on all the time when your Beeline is on your bike then your Beeline needs to be calibrated. To calibrate your Beeline, follow the instructions here
2. If the symbol is coming on from time to time it is due to objects or buildings nearby that have strong magnetic fields. Just keep cycling - your Beeline will correct itself as soon as you are away from the disturbance