A "heat engine" is a machine which converts internal energy (from a high temperature body) into some other form of energy.

When the lid on a pan of boiling water is lifted by the steam inside, the internal energy of the steam is being converted to gravitational potential energy of the lid. This is a very simple example of a heat engine.

The conversion of energy from some other form of energy to internal energy of a substance can be done with 100% efficiency of conversion. For example, 100J of electrical energy will be converted to 100J of internal energy by a resistor.

Conversion of energy from internal energy to some other form cannot be done with the same efficiency.

To show this, consider the possible heat engine shown below. (The diagrams are very simplified but are similar to some real heat engines).
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If we simply push the piston back up we will do just as much work as we obtained during the expansion stage: not a very satisfactory situation!

To make sure that we do less work pushing the piston back to its original position we could allow the gas to cool down before pushing the piston up. Or we could let this still quite hot gas escape, push the piston up and then put in fresh gas.

However you do it you will always find that you need to allow the gas to give up some of its internal energy to the surroundings (as it cools down). Therefore, the conversion of energy from internal energy of the hot gas can never be 100% efficient.

This is one illustration of the second law of thermodynamics which can be stated in many different ways. One statement is as follows
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The hot body is called the heat source and the cold body (often the surroundings) is called the heat sink.

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