i feel i have a pretty good understanding on string theory so i'll try and give it a shot
the average person believes everything in the universe is made up of atoms.
everything in the universe
is made up of atoms, however what makes up the atom?
protons, neutrons, and electrons of course, and to most it ends there, but to those know a little more, they realize that protons and nuetrons are actually made up of smaller particles as well, called quarks, in one point of time people thought atoms were as small as it gets, but as time went on we discovered the electrons, protons, and neutrons, and as time went even further on we discovered quarks, now does it stop there? or does something make up quarks? currently we do not know but some beleive that even quarks may be made up of smaller particles, and those particles made of even smaller particles, and so on untill we get to the utmost fundamental particle in nature, a particle which can not be broken down anymore, a particle that in all essense is not even a particle
the common theory is that matter is made up of point particles, however string theorist beleive that all matter is actually made up of tiny strings of energy, to imagine what a string may look like, you can picture an elastic band, only instead of rubber think pure energy, and shrink it down a billion billion billion billion times or so
now if everything is made up of strings why isnt everything the same? its made of the same things right? well yes it is made up of the same things but the strings that make it up vibrate in differnt patterns, the vibration these strings give off determines what the string "is" in our reality
string theory requires 11 dimensions because the strings need the extra dimensions to vibrate, 3 dimensions would not give the strings enough diversity, and with these extra dimensions comes the "calabi-yau" shape, this is the shape the extra dimensions are theorized to be curled up in to assist the strings vibration, these extra dimensions are so small that attoms can not pass through them
the importance of string theory is that , if it is true we are one step closer with developing a theory of everything, it also means that multiple dimensions and spacial membranes are a possibility, leading to explanation of the big bang

any questions feel free to ask
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This is good. But the string theorists have suggested no experiments, and a theory without testable consequences is just a fairy tale.
However, I like the fact that if it were true, it would solve the problem of singularities, in Black Holes, at the Big Bang, even inside an electron. It also has one remarkable consequence, these extra spatial dimensions. According to some versions (Stephen Hawking's, for instance) at least one is unrolled, linear and infinite. It is all around us, but physical matter cannot see it. Mind can, however, since it is dark matter. Such extra infinite spatial dimensions are required to understand apports (sometimes called teleportation).