It means knowing the very basics. The English alphabet begins with the letters A B C and it's something you will learn as a very young child so it is seen as being easy.

The ABCs are the basics, i.e. the simplest and most important parts of something. For example, the ABCs of math are addition, subtraction, multiplication, division and square root.

thank you Jesse=)

I apologize for my typing mistakes. It should say, "(In this case, the meaning* of....)

It means the basics of something. Or, in other words, the fundamentals. It also means the beginning steps of something. For example, the ABCs of the United States of America are life, liberty, and the pursuit of happiness. (In this case, the means of "the ABCs" is "the basics.")