In plain English
A chatbot is an application that conducts a back-and-forth conversation with people in everyday language, by text or voice. Older chatbots followed scripted rules and decision trees. Modern ones are usually built on a large language model, which lets them respond far more flexibly and naturally to whatever you type.
A worked example
A bank support chatbot that answers questions about your balance, helps reset a password, and hands you to a human agent when the issue is complex.
Common confusion
A chatbot is the product you talk to, not the model underneath it. The same underlying model can power many different chatbots with different roles and personalities.