You can't collapse other posters' writing without abusing the ignore function (which will throw you back to the start of the thread anyway).
You can collapse your own text, however, using the spoiler function. I've found this can be a useful tool if you're posting a wall of text (sometimes relevant for streams of data on election nights).
Also, if you're writing a long post, and want to avoid clogging up the post, using the spoiler headings as chapter titles can allow you to keep the long-post easily navigatable.
If you open the "quote" option on this post you can see the relevant bbcode below:
Spoiler alert: Explainer
See, walls of text can be easily hidden.