What does corruption of a minor mean?

It can mean a couple of different things. One of them is aiding or encouraging a minor (someone below the age of majority, specified by law and different from place to place but generally somewhere between 18 and 21) to commit an illegal act. Fagan from Oliver Twist is an Excellent example of someone who would be guilty of this. The other is sexual activity with a minor (in this case, someone below the legally specified age of consent, which is often lower than the age of majority). It's similar to statutory rape except that statutory rape usually proscribes only certain specific acts while "corruption of a minor" is more of a catch-all covering a much broader range. Also, it usually carries a significantly lower penalty.