A reservoir is a supply or source of something (think like reservoir lakes that have water for us to drink). Water is often used as a heat reservoir because it can hold a lot of heat. This is hard to grasp, but one way to think of it is that water can take in or gave out a lot of heat without changing its temperature a lot--it takes a lot of energy to heat up or cool down water. For example, if you put a sealed bucket of hot water and a sealed bucket of hot air in a room, the water will stay warm much longer than air. That is because it contains a lot more heat that it has to give off to lower the temperature. So this "heat reservoir" gives off a lot of heat for a long time. The opposite is also true: if you heat water up to a boil it takes a long time, you have to put in a lot of heat from the stove. On the other hand, the air around the burner on the stove immediately gets really hot because the air "holds" less heat as it increases in temperature. Air is not a good reservoir of heat.

This is related to a property called "specific heat."Specific heat is the amount of energy you need to raise the temperature of one gram of a substance 1 degree Celsius. For example, it takes about 4 joules of energy to raise water 1 degree Celsius. This is pretty high. Also this amount of heat energy is called one calorie (like in food). You'll learn about this in high school if you take Chemistry.