In One Sentence
LLM (Large Language Model) is an AI model trained on a massive volume of text from across the internet, capable of understanding and generating natural-sounding language like a human. It is the "brain" behind ChatGPT and Claude.
What does this look like in practice?
For example, ask ChatGPT "Tell me about Edo-period food culture" and it returns natural-sounding writing as if a human had answered.
That is because the LLM inside ChatGPT (specifically GPT-4 and the like) is "predicting" and generating the answer best suited to your question from the trillions of words it has seen during training.