The UK, like the rest of the world, generates and distributes AC power. AC is short for alternating current, and we use this because it’s easy to generate, switch and change the voltage. We use most of our power at low voltage but generate and distribute at high voltage because it’s more efficient to do so.
Alternating current means the supply voltage rises from zero to a maximum, drops back to zero and then does the same thing again in reverse. Conventionally we call the two ‘directions’ positive and negative. The mains supply here is 50 cycles a second and a cycle is one positive and one negative. The shape of the voltage rise and fall is sinusoidal so you can visualize one cycle as halt an ellipse going positive followed by one going negative.
Simplistically, think of a generator as a coil of wire rotating between two poles of a magnet.