What is meant by the term “polarity”?

The 'polarity' of the voltage at some point in an electrical circuit is either
positive or negative, compared to some other point in the same circuit.

If any point in the circuit is connected to "ground", then all of the voltages measured anywhere in the circuit are positive or negative compared to
ground.

'Conventional' current in a circuit always flows from the positive point
to the negative point, although we understand now that the things that
are really flowing are electrons, electrons have negative charges, and
they actually flow from the negative point to the positive point.