What is the compound word for a explanation at bottom of page?

There isn't a single, established compound word for an explanation at the bottom of a page.

However, you could use these options:

* Page-bottom explanation: This is the most straightforward and descriptive option.

* Footnote: This is a common term for an explanation placed at the bottom of a page, often marked with a superscript number in the text.

* Endnote: Similar to a footnote, but typically placed at the end of a chapter or document.

The best option for you will depend on the specific context.