"Placeholder" stands for whatever you want to name the tag! For example, when I'm doing collapsible sections, I write things like "Appearance," "Personality," etc. It's mainly to distinguish multiple click-to-open functions on one page from each other.
<div class="mw-customtoggle-placeholder">The thing you want to click here</div>
<div id="mw-customcollapsible-placeholder" class="mw-collapsible mw-collapsed">The content you want to expand here</div>
