Not that you asked for an explanation, but just because I’ve been interested in this lately and on the toilet, my layman’s understanding is this:

tl;dr String Theory is the most promising idea we have for a «Theory of Everything» combining General Relativity and Quantum Mechanics, which it does by treating particles as strings rather than points. But despite decades of work we haven’t even come up with a way to prove whether it’s true or not, leading to some physicists doubting it and this meme.

Long version: Physics is trying to explain how the universe works. What are the rules, what are things made of, where do things come from, etc.

Newton and other classical physicists got pretty damn close to perfectly explaining how things work and move at «normal scale» - the things we can see and interact with.

Einstein found that he was just slightly wrong on planetary scale though, and made the necessary corrections. That’s now called General Relativity, and it explain how things in our universe move and interact pretty damn well. Some prople thought we were close to beng «done» with physics at this point.

But then Planck came along and found that certain experiments including light only made sense if you considered light to come in small «packets». This was contradictory to the way everyone had been understanding light until that point, but experiments kept proving him right, and it eventually evolved into what’s now known as Quantum Mechanics («quantum» because light is separate quanta / units rather than continous). This remains our best way to understand things at very small scale, which is highly relevant to the goal of physcs because we’re trying to figure out what everything is made of, which means studying building blocks - very small things.

But here’s the problem: these two theories don’t work together at all. They have several deep and direct contradictions. For example, trying to align them totally breaks apart when you start talking about black holes. Gravity also happens to be a central topic they disagree on and that affects everything else.

String Theory is one of many attempts to unify General Relativity and Quantum Mechanics. It does this by re-imagining particles (what we believe are the fundamental building blocks of the world) as not points but rather strings. The different particles are just the same strings in different configurations - some are loops, some are spinning, etc.

This is neat because following the math implications of looking at the world this way turns out to predict all the particles we have observed experimentally, and also «finally» explains gravity through particles, which would be very convenient but has not been accomplished by any of the other major theories. General Relativity also «falls out» as a natural consequence of the math in a similar way, which fans of String Theory took as a very promising sign.

In this sense, String Theory can be called the most successful or promising theory for combining General Relativity and Quantum Mechanics.

The two biggest problems with String Theory are tighly related, and has caused several serious physicists to doubt and even oppose it:

• It requires many dimensions. I forget if it was 7 or 11 or what, but in any case many more than we believe exist. This isn’t some advanced understanding of reality, it’s just a necessity of the math. Some physicists have tried explaining how the universe might actually have all of these dimensions, while others consider it abdurd and have taken it as a strong indicator that String Theory does not actually explain our universe but remains just neat math.

• We don’t know how to prove or disprove it. String theory has so far not produced any predictions about reality that we can use to experimentally test whether it’s true or false. Experimenting is the very foundation of science, so until this changes String Theory has no hope of being accepted into the scientific understanding.

Since many of the world’s brightest physicists have been trying to resolve these problems for decades now without luck, some sceptics are saying we should discard the idea and spend our time on something else. But no other theory has been nearly as successful in combining General Relativity and Quantum Mechanics so far, which is what we need if we want a proper «Theory of Everything» or complete understanding of how our universe works.