LLM based AIs have lots of "features" which are kind of synonymous with "concepts" - these can be anything from `the concept of an apostrophe in the word don't`, to `"George Wash" is usually followed by "ington" in the context of early American History`. Inside of the LLMs neural network, these are mapped to some circuitry-in-software-esque paths.
We don't really have a good way of understanding how these features are generated inside of the LLMs or how their circuitry is activated when outputting them, or why the LLMs are following those circuits. Because of this, we don't have any way to debug this component of an LLM - which makes them harder to improve. Similarly, if LLMs/AIs ever get advanced enough, we'll want to be able to identify if they're being wilfully deceptive towards us, which we can't currently do. For these reasons, we'd like to understand what is actually happening in the neural network to produce & output concepts. This domain of research is usually referred to as "interpretability".
OpenAI (and also DeepMind and Anthropic) have found a few ways to inspect the inner circuitry of the LLMs, and reveal a handful of these features. They do this by asking questions of the model, and then inspecting which parts of the LLM's inner circuitry "lights up". They then ablate (turn off) circuitry to see if those features become less frequently used in the AIs response as a verification step.
The graphs and highlighted words are visual representations of concepts that they are reasonably certain about - for example, the concept of the word "AND" linking two parts of a sentence together highlights the word "AND".
Neel Nanda is the best source for this info if you're interested in interpretability (IMO it's the most interesting software problem out there at the moment), but note that his approach is different to OpenAI's methodology discussed in the post: https://www.neelnanda.io/mechanistic-interpretability