Generative AI tools are those that produce text, code, audio, images, and other media based on a user's prompting. Some examples:
These tools have been trained on large amounts of data. Training refers to passing data to a model so that it can extract features related to the data and make connections between data points.
With a tool like ChatGPT, a similar premise exists. A very large amount of text (about 570GB of compressed plaintext data for GPT-3, according to Brown et al., 2020) has been given to this model, so when you ask it a question it is able to decide what a highly probable reply would look like and responds with this very real seeming answer.