Meaning of negative voltage
Voltage is measured relative to something.

Say you measure the speed of a train coming toward you. If it is approaching at 50 km/h, you could express that as a positive value: +50.

If you again were to measure its speed as it leaves, from the same point of reference you would say it is departing at 50 km/h, but you could express this as a negative value: -50.

Voltage is the same way, depending on what your reference point is to ground, you might measure voltages as being either positive or negative.