Like any pressure, when you measure voltage, you are always measuring the difference between one voltage and another.
There is no absolute voltage. Absolute zero volts has never been defined.
Ground is an arbitrary voltage that we use to measure other voltages from.
Ground is zero volts, but it is only zero volts because we have
chosen to say it is. If you find a voltage that is lower than what you
decided to call ground, that is a negative voltage.
Positive and negative voltage are not opposites.
A negative voltage is just some voltage that is lower than some
other voltage that you have agreed to say is zero volts. This is the
same as a negative altitude that is only negative because it is lower
than sea level, which people have agreed to call zero (for no better
reason than why you may call some arbitrary voltage ground). The floor
of Death Valley is at an altitude of -280 feet, but rocks do not fall
up if you drop one there. It is only a negative altitude because it is
lower than sea level which is just an altitude of zero because we have
agreed to say it is.
Later, we will see voltages that seem to be both positive and
negative at the same time. No confusion here. That voltage is positive
to some lower voltage and negative to some higher voltage. That’s no
different than the floor of Death Valley. It is lower than sea level
but higher than the Dead Sea. So, to sea level it is negative and to
the Dead Sea, it is positive. Voltage can be the same way.
Current is the flow of electricity through wires.
The actual electrons flow from negative to positive (electron flow, electron current),
Academia and the electronics industry as a whole usually
pretend that electricity flows from positive to negative (conventional
flow, conventional current).
It is easier to visualize how circuits work using the
conventional flow model (electricity flows from a higher voltage to a
lower voltage, etc.).