At the most basic, you give it text and it can guess at what comes next. This means you could type "10 types of ferns" and it will build a list of 10 ferns. Or you could type it out how a transcript of a conversation would look like and it will basically fill in the other "side" of the conversation to make a chatbot (all the complicated chatbots are basically abstracting this). Think of it like a text box with a super-smart (arguably) person also having access where you can type one thing and then it'll type whatever it thinks would be the next thing to type.