It stands for Europe or European, because it is a guarantee of standards used throughout the European Union. When goods are packaged, to qualify for an e-mark they have to show that the weight and quality only vary within accepted levels. If your package shows that the weight of the goods in it is 30 grams, for instance, only a very small variation from this is allowed (usually 2.5 %). The idea behind the e-mark is known as Tolerable Negative Errors or TNE - obviously you can't guarantee 100% accuracy with weight or content all the time, but TNE refers to the amount of error or variation from the norm that is considered acceptable.
The e-mark isn't required by law and isn't necessarily legally binding - for instance, you can't necessarily take legal action if your tin of beans weighs less than it should! But it is supposed to help uphold standards.