system
Given the following post, respond as if you're commenting on Reddit:
human
What string theory is...
gpt
Err - yeah, what SirTrumpalot said.

String theory is very complicated and has nothing (particularly) to do with strings.

So imagine that you have a lava lamp.  These are fascinating objects that are lamps that cause bits of 'lava' to go up and down in a jar.  It's very easy to describe the lamp by its dimensions (height, width, depth) to give an impression of what it looks like.  You could even describe the lava within the lamp like this.  Except of course the lava changes over time, so you have to add in another dimension of the time that the lamp was at its dimensions that you just described.

It turns out that your lava size and shape at any particular time also depends on its temperature (hotter lava rises to the top as it loses density compared to the liquid around it, whilst cooler lava falls to the bottom of the lamp - where it is heated by the lamp again).  So now we have an added dimension of the size and shape of the lava - its temperature.  So we have 5 dimensions already that describe the lava: height, depth, length, time, temperature.

It turns out that this lava lamp is a magic one that changes colour as well, apparently randomly.  So to describe it you also have to describe it in terms of its colour, giving a sixth dimension: height, depth, length, time, temperature, colour.

And you can keep adding these 'features' to the lava lamp to keep coming up with lots of new 'dimensions'.  And this is what string theory is.  It describes the world in lots of different dimensions, some of which we don't ever notice changing, some of which we don't even know what they are (eg if your lava lamp also had a feature called 'galumph' and it changed over time, you could describe it by its 'galumph').

EDIT: Three 'it's' to 'its'