String theory is based on the fact that everything does not consist of tiny zero-dimensional points, but of the smallest one-dimensional strings. It is one of the dominant theories of our time. It also explains many phenomena with additional dimensions. Ultimately, humans only perceive 3 dimensions of space, plus the supposedly one-dimensional time. However, string theory solves problems based on 13 or higher dimensional geometry. Ultimately, the problem of whether the point has an extension is an older problem. So the question arises, why there are infinitely many points between 0 and 1 on the number line, but these supposedly have 0 dimensions and 0 extent, but still fill this area. This problem would be solved with one-dimensional strings with one extent. Here you can find out more about Dimension.
Frequencies are also important for string theory, so mass and all other physical phenomena should arise from the oscillation of these elementary strings. In the area of elementary particles in particular, many problems can be solved with string theory.
I can recommend these books to a limited extent. If you read a lot and want to know a lot about it, Brian Greene and Lisa Randall are in good hands. Personally, I prefer Albert Einstein's brevity, but these are standard works of string theory. I don't know Brian Greene's latest book, but it's stylistically just as good as any of his others.