Charge means that the body experiences a force in an electric field. A charge generates an electric field, which generates a force on other charges particles. Two bodies are said to repel if they force each other away and two bodies are said to attract if they force each other closer together.
Now, I'm not really answering your question here of "why," I admit that. The problem is, it's not really possible to say why something like charge works the way it does. We can describe it using mathematical structures that represent (and even predict) the behavior of fundamental behavior of quantum objects, but when you start asking "why," well... They're called 'fundamental' because that's just the way it is. There's no reason like electric charges repel and opposite charges attract that we can even find a hint for. But they do, and we can use that.