Do you mean the OVERALL premise of the entire series? I can help you.
Ages ago, somewhere out in the universe, on a planet called Gallifrey, there lived a humanlike race that was sort of like the Vulcans: highly intelligent and talented, but rather coldly intellectual instead of having as much sentimental feeling as we have. Their natural lifespan was already much longer than ours; but they discovered a way to give themselves a still longer life, by being able to RE-CREATE their own bodies when they died of old age or by violence or accident. This is called "regeneration." They can only do this twelve times per person, making thirteen lifetimes EACH OF which could be a thousand years long. Only in the first "life" do they have a childhood; in each regeneration they re-appear in a new ADULT form. Until they run out of regenerations, the only way a Gallifreyan can permanently die is if he is SO completely wiped out of existence that there's nothing LEFT to regenerate.
The other big achievement of the Gallifreyans was creating all-purpose vehicles, which they call simply "capsules," but for which The Doctor coined the name T.A.R.D.I.S. for the benefit of English-speaking Earthpeople. That stands for Time And Relative Dimensions In Space. Each T.A.R.D.I.S. can travel to literally any location in time and space. It was this invention which caused the Gallifreyans to become known as the Time Lords.
Now, concerning The Doctor himself. His real name, according to one episode, was Theta Sigma; but he didn't like his given name, just as a human might resent being named Irving Squigglepoof. Theta Sigma lived most of his first life as a more or less normal citizen of Gallifrey; thus, when he decided he wanted to leave home and explore the cosmos on his own, he looked like an old man--yet with no regenerations used up yet, he was actually still fairly young by Time Lord standards.
A young Gallifreyan woman, also bored by the overly-formal Time Lord culture, departed with The Doctor--as he began calling himself. They went to Earth, both because The Doctor found it interesting, and because they looked like humans and so could hide out there. They were pretending to be a grandfather and granddaughter (she took the name Susan), living in twentieth-century England, at the very beginning of the series. The earliest episodes concentrated on TIME travel, not on going to other planets.
Since then, viewers have learned that you can hardly walk across a street in the "Doctor Who" universe without stumbling over six or seven super-duper-unbeatable space aliens, EACH of whom claims to have created all of human civilization.
Does that help?
