Deep learning is machine learning with large, multi-layered neural networks. The 'deep' refers to many stacked layers that learn increasingly abstract features — from edges to shapes to objects in images, or from letters to words to meaning in text.
Nearly every modern AI breakthrough — language models, image generation, speech recognition — is deep learning. Understanding the term separates the technique from the marketing around it.
A deep network trained on millions of photos learns to recognize cats not from rules about whiskers, but from statistical patterns discovered across the layers.