See, in our life, all around us, we experience only 3 dimensions. Length, breadth and height. If you plot these three in a three dimensional graph, with an x, y and z axis, you'll find that everything in the World has it's own coordinates. Like the place you're sitting right now has its own coordinates, and since you're sitting there, nobody else can sit there. Which means, a set of unique coordinates can't be shared by two things (in language of Physics, you're pretty much a thing). Or in other words, no two objects can occupy the same space.
However, according to calculations, we can surmise that there may be other dimensions too. String theory predicts Eleven dimensions in total. Time is the most accepted fouth dimension. Why?
Say you're sitting on a chair in the afternoon. Now, at night, you leave, and your mom sits there. See how the same coordinates are being occupied by you and your mom? Only one thing has changed-Time.
Now, what if, in the other, higher dimensions, the coordinates change based on our choice? Let me explain. You're sick today, and you don't wanna go to school. So you stay home. However, there was an equal probability that you could've gone to school. Mathematically speaking, there is no reason why staying at home should be preferable to going school, right? So, it is understood that you did go to school, just not in this universe.
1.Conceptial picture.
Now this was a simple example. It has only two possible outcomes. Let's take it up a notch. Say a lottery is held, and 1 crore tickets are sold. Now each of those 1 crore ticket buyers have an equal chance to win. And each of them DO win, in the respective 1 crore universes. By this rule, there should be infinite number of universes, all subtly different from each other. If there are really 11 dimesnions, which seems pretty wacky now, then 10 of those could be equal, and if the last one is even slightly different, you get a whole other universe! Which probably means that someone exists right there in front of your nose as you're reading the answer, only your senses can't percevie them.
The concept takes a little time to digest, so I suggest you go ahead and learn more about it. Once you do, you'll know how amazing Physics can be! Cheers!