A question about the operation of Jupyter notebook

Near the end of the first half of the lesson 2 video, at 48:23:
48:20 - Introducing the Kaggle State Farm Competition

I noticed Jeremy can hide the content below a header and later on he can expand it, by just clicking a reversed triangle in front of the header.
Does anyone know how can I do this, something special in the markdown header?

First post see jupyter extensions About the Computational Linear Algebra category