The oceans absorb a great deal of heat energy from the Sun. As the Earth turns, the wind blows, and the Sun and the Moon pull on the Earth (and the Earth's oceans), the oceans circulate around the Earth. These circulations are called gyres. Warm parts of the ocean – the top 400m or so - move. The heat capacity of water is high, so a lot of energy can be stored, and much of this energy is still there when the water has reached the coldest part of it's cycle.
This means than otherwise cold parts of the globe (like Britain) are warmed by ocean currents and made more temperate than they would otherwise be. The current that warms Britain is called the 'Gulf Stream'. Without it, Britain might have a climate and temperatures closer to those experienced in Siberia.