String theory posits a universal underlying 'theory of everything' which is still undergoing development.
The physicists propounding it (and Susskind is one of the primary proponents), feel they've mostly 'solved' the 'everything' except for gravity (which doesn't seem related and they can't find particles for it), 'dark energy' and 'dark matter' (they can't account for how both came to exist); and, for the interactions within the nucleus of an atom. The basic idea is founded on quantum mechanics, which is basically that everything's composed of 'particles' which can be quantized
(measured discretely) which aggregate via oscillation or wave action (which the 'particles' ride or drive or are attached to), thus forming 'strings' and hence the 'duality' of their acting like both waves and particles. The idea is to map out how these particle-waves join and separate to create everything in the universe, from the smallest starting atom to the grand spectrum of the universe with its planets and maybe.. alternate universes or mini-universes which co-exist right alongise or even within, ours.
The foundation of these ideas goes all the way back to Newton, who posited Light as a wave but also a particle. Scientists after him came to favor 'wave' until Einstein asserted also 'particle', and then with further study and experimentation in the 1900s, they came to understand that EVERYTHING is essentially the product of a wave action: light, sound, radio waves all being part of a single integrated system which by interaction CREATES matter in small and in great sizes over time
The 'M' theory is a version of string theory which posits due to certain math findings in string theory, that these 'strings' are or create, 'dimensions' beyond what we can humanly detect: the number of 'dimensions' vary a lot, and Dr Susskind at the end of the videos below, briefly explains how you ferret out how many, but it's just basic intro.
These lectures are math-heavy. If your eyes glaze over with math, keep using 1Jn1:9 as you do some exercise or housework while you listen, cuz every five or 10 minutes, Dr Susskind summarizes the import of what he math'd out. I even sleep to these lectures, just to get familiar with the vocabulary (how you learned language and math as a kid, really).Basic intro lectures
which focus on concepts more than math, start at video 93-101 in my playlist (I might move them to other locations earlier in the list, and will edit this post if I do to the revised location):https://www.youtube.com/watch?v=NZ-ElsvYKyo&index=93&list=PL559FEFBECB9CE9FA
The advanced lectures
are in videos 62-71 in my Physics playlist:https://www.youtube.com/watch?v=25haxRuZQUk&index=62&list=PL559FEFBECB9CE9FA
Updates for newer discoveries are in videos 102-111 in the playlist, but again are mostly conceptual, less math-heavy, and general, basic discussions:https://www.youtube.com/watch?v=2eFvVzNF24g&list=PL559FEFBECB9CE9FA&index=102
As a result, there is now a 'Standard Model' of how these particles work, and its lectures run from video 112-121 in the playlist:https://www.youtube.com/watch?v=Igl8hE3Eac0&list=PL559FEFBECB9CE9FA&index=112
For his explanation of the Gravity problem, click here
As for the problem of dark energy and dark matter, a shorter video on that (see it in Youtube for related longer vids):https://www.youtube.com/watch?v=XUQER8r-ROc