In plain English
A large language model is a type of AI trained on enormous quantities of text so it can understand and generate human language. 'Large' refers both to the volume of training data and to the number of internal parameters, often in the billions. It works by predicting what comes next in a sequence of text, which turns out to be remarkably effective for writing, summarising, translating, and answering questions.
Why it matters
LLMs power most of the AI assistants people use day to day, so understanding their basic mechanism, predicting likely text, explains both their strengths and their failure modes.
A worked example
ChatGPT, Claude, and Gemini are all built on large language models, which is why they can draft an email or summarise a document on request.
Common confusion
An LLM does not understand language the way a person does. It is predicting statistically likely text, which is why it can be fluent and wrong at the same time.