A chatbot is a program that communicates with people using text or voice. It usually helps with simple tasks - answering questions, making reservations or dealing with customer support queries.
It used to work mainly according to pre-written rules (e.g. "if a user types A, answer B"). Today, modern chatbots use various forms of artificial intelligence, and increasingly they are large language models (LLMs), so they can respond more intelligently and naturally.
The important thing is that chatbot ≠ AI. Most web chatbots still work without using AI.