You can use Markdown and HTML.

For example, if you add a Markdown cell (Cell → Cell Type → Markdown) then you can add headings

# Chapter 1
Some text.
## Subsection 1.1
Some more text.

which will render approximately like this:

Chapter 1

Some text.

Subsection 1.1

Some more text.

but the Jupyter notebook also adds links which you can see by hovering over the headings.

Then you can link to these headings in a Markdown cell using the link syntax:

See [Subsection 1.1](#Subsection-1.1).

By using HTML you can also link to places which are not headings, e.g. if you make a div with an id

<div id="Example-1">
<b>Example 1.</b> $3^2 + 4^2 = 5^2$.
</div>

then you can link to it by [Example 1](#Example-1) in a Markdown cell.