What means the "collapsed": true parameter in markdown context?

In Github diff, I see sometimes changes like this for Jupyter notebooks:
And I could not find what the collapse parameter means in markdown context:
https://nbformat.readthedocs.io/en/latest/format_description.html#markdown-cells
Can someone tell me what this is used for?

Start with the markdown-cells section of the nbformat format description and compare it with the `collapsed` field shown in the linked GitHub diff. Done means clearly explaining what the parameter controls and where it is used in notebook markdown cells.

Written by the indexing model from the issue text.