This is a wonderful question - but quite difficult to answer. Modern Biology tells us that human beings are fairly new to the planet. There's been life on Earth for around 3.5 billion years, but that life has constantly changed form through the process of natural selection and human beings weren't around for most of it.
In a very condensed nutshell it goes something like this: imagine you have a creature representing a tree trunk. Over time, the children of these creatures are born slightly different to their parents, giving rise to new varieties of creature, and so the species splits into several categories, like the boughs of a tree. Each of these new species then does the same, giving us lots of new creatures again (the branches) and so on (twigs) and so on (twiglets). Give this process a few billion years and what began as one species of simple bacteria has turned into the millions of different species we see today, one of which is the human race.
Imagine if you began at the present day, with the human race, and traced back along the family tree, going right the way back to the first creature on the planet. You could set up a timeline showing the gradual evolution of humans. But at what point did we become the way we are? If our oldest ancestor was some slimy bacteria floating in a rock-pool, when do we draw the line and say "this is when the human race began?"
It's surprisingly tricky to give a good answer because the creatures which came just before humans were about 99.999% identical. In fact, within the last few thousand years, we've seen changes in the shapes of human skeletons, so are we a different species to the humans who lived in ancient Greece? We are slightly different after all. But if you want to get technical, I am sliiiiiightly different to even my own parents. My genetics are not quite the same as theirs due to tiny mutations, would I be classed as a separate species? This is the problem: which tiny change was the one which made us "human". It would be like taking a grain of sand, adding another one, adding another one, and then asking when it became a pile. There is no obvious point, and different people would draw the line in different places.
After all, do we consider human to be when we began walking on our hind legs? When we began developping language? When we invented culture? When we began creating art? It's a good issue to debate, but most Biologists would agree that the human race (under any of the reasonable definitions) emerged somewhere between 1 million and 500,000 years ago.
In 1995 a woman named Nancy Lieder claimed she had been contacted by aliens and warned of an imminent collision between the Earth and another planet named either Nibiru or Planet X. The name Planet X comes from Percival Lowell, the astronomer who suggested an extra planet beyond Neptune (which he nicknamed X for short).
This claim from Nancy Lieder is the sum total of the so-called "Planet X theory". It's really a bad name, because a theory is an idea which has lots of evidence to support it, but Lieder's claim has none. In fact, Lieder's original claim was that the Planet-X collision would take place in 2003...which I'm pretty sure was proven false. There are still some people who claim that the Planet X event is going to happen, but these claims aren't based on any actual data so there's little reason to be alarmed by them. I could just as easily claim that a giant strawberry is headed for Earth. Technically you can't prove that false, so I could refer to it as the Stawberry-event-theory, but there's no reason you'd believe it. So, what is Planet X "theory"? Nothing to worry about.