GPT stands for "Generative Pre-trained Transformer," and it names a family of large language models that generate text. Breaking down the name explains a lot: generative means it produces new text; pre-trained means it learned language patterns from a huge body of text before you ever used it; and transformer is the neural-network design, introduced in 2017, that made modern language models possible.
Because GPT-style models became so well known, people sometimes use "GPT" loosely to mean any AI chatbot. Strictly, it refers to models built on this particular architecture. The takeaway for everyday use is the same as for any language model: it is a powerful drafting tool whose output you should still review and fact-check.