Deep learning is a branch of machine learning that uses neural networks with many stacked layers — that "depth" is where the name comes from. Each layer learns to detect increasingly complex patterns: in an image model, early layers might spot edges, later ones whole objects.
Deep learning is the engine behind the recent leap in AI ability — the language, image and voice models that suddenly work well are almost all deep-learning systems. It became practical thanks to huge amounts of training data and powerful hardware. The trade-off is that these models need a lot of data and computing power, and their inner workings can be hard to interpret.