What is string and M theory?

String theory says everything is a vibration in various dimensions. Obviously, it has to be something that's vibrating so it's said to be energy in the form of strings, there has to be ten dimensions to explain all the sub-atomic particles that we have but there are five competing versions of string theory. In M-theory, an eleventh dimension is added and all the strings are said to be portions of a membrane through this 11th dimension, with M-theory, all five of the string theories collapses into the same theory.

This is a rather complex subject. It's difficult to explain to the uninitiated, but I will try to keep it simple. Not sure its possible, though. LWell, in physics there a four "forces" which govern the universe and how it works. The electromagnetic force is what allows the atoms which you and I and everything else to stay together. The strong force holds the nucleus of the atom together, and the weak force allows radioactive decay. And finally, the most well known and even palpable force, is gravity.

How ever, gravity is the weakest of the four forces, and this creates a big problem for physicists who want to come up with a grand unified theory that reconciles gravity with the rest of the forces. The GUT theory (grand unified theory) is the holy grail of astrophysics and when we apply gravity to our 4 dimensional universe, we find it so incredibly incompatible with the other 3 forces, that physicists have to introduce up to 11 dimensions in order for it to be one cohesive physical law. These dimensions are absolutely undetectable other than the gravity that they emit. Within these dimensions are various "strings" which "vibrate" at different frequencies. The strings can be open-ended, closed, looped, etc. Depending on the frequencies on which they vibrate different particles are produced. One of the strings may indeed create the griviton, the particle gravity might be made of.

An earlier proponent of M-theory is based on the possibility of massive"membranes" or branes as they are now called, which separate differenth universes. These branes "ripple" and when they these branes make physical contact with eachother, gravity is leaked into our universe. Which is possibly why gravity is so weak.

It's a bit more complex than this, but these are just the basics.

It would take a while to explain and it's difficult to do so in simple words without oversimplifying it.

String Theory proposes that matter is comprised of tiny vibrating strings of energy. If you could tell us what you don't understand it would increase your chances of getting an answer that you're looking for.