erm...ok....
The end of it is that John Connor is a leader of a resistance of humans against machines in a big futuristic war.
Terminators (good and bad) are sent back in time from then to either protect or kill him depending on which side sends them.
Confusingly though in the first film, John's mother, Sarah Connor is actually impregnated by a human sent back to save her from a terminator (trying to kill her to stop her getting pregnant with John Connor) but the human ends up being John Connor's father.
The first 3 films are about the terminators sent back to stop/save Jon Connor, the next one is about the future itself and the war between humans and machines.
Does that make sense?