The most basic, nitty-gritty theory physicists have for explaining the Universe is called Quantum Field Theory - it's the deep-down set of laws we assume everything else is based on. Like learning the basic moves of a game of chess, if we understand the laws of QFT we can explain any phenomenon you care to mention.
There are just two problems. QFT doesn't explain the existence of gravity and QFT leaves an enormous mystery open: why is nature so messy? What I mean is that the raw ingredients of our Universe don't follow any logic, they're a higgeldy piggeldy mess of particles whose properties seem to be random.
It's like discovering that the ingredients of the Universe are a bag of revels: some large, some small, some with caramel centres, some Malteser etc. There's no reason to it but it definitely seems to be true. But since QFT can't explain this wierdness, QFT must be incomplete and we're on the lookout for a new theory to explain reality. One of the possible avenues is an idea called string theory.
In the 1970s a group of theoretical physicsts began tackling the QFT problems in a casual way - mainly out of curiosity and for fun, they began imagining that the deep structure of the world wasn't made of fields and their particles, but made of strings. Everything in the Universe was really made of a tiny string-object and all the particles we can see are just these strings coiled up and vibrating in different ways.
The first string theories were only used to deal with a small part of QFT however, what's called the bosonic aspect, so the theory's full name was Bosonic String Theory. It was never intended to be rigorous though and was mainly developped as a way to keep the equations simple. In fact, Leonard Susskind (the theory's main inventor) has even described it as something "we were just playing around with", but then something rather weird happened - bosonic string theory turned out to accidentally explain the existence of gravity.
Suddenly, people got interested and wanted to see if they could use this strange new idea to explain all of QFT, not just the bosonic part of it. Could we use string theory to maybe explain all the particles in the Universe? One of the tricks used was to assume that strings could act in symmetrical ways - vibrating left, but also vibrating right - spinning clockwise, but also spinning anticlockwise (or something similar to that, the reality of what the strings are doing is too hard for humans to visualise).
By imagining that all these string-like objects had a property called "supersymmetry" Physicists discovered they could explain quantum field theory and include gravity, as well as giving us an idea of where the randomness of particles comes from. So the strings were renamed "Superstrings" and the idea is still being worked on today!