In a journal, what's the difference between an "article" and a "monograph"?

The word monograph is derived from "mono" and "grapho", so the implied literal meaning is "writing on a single subject". Monographic sections are thus sections linked by a single theme, and often only have invited contributions. In the specific journal issue, the theme appears to be related to monuments/architecture from the Almohad Caliphate.

A journal that is clearer about what they mean with monographic sections is Kriterion Journal of Philosophy:

Monographic sections are small collections of original research papers on a specific topic, with an introductory part, that are, in contradistinction to special issues, published as part of a regular issue together with other articles.

My general analogy would be that article : monograph : book :: short story : novella : novel. It's a distinction based on length and scope. (Monographs and novellas are probably harder to get published than their more common kin.)

Looking into that journal's submission policies, it appears that articles have a word limit while monographs do not. However, in the issue you cited, some of the articles were longer than the monographs, at least in pages.

It appears that they ask for / commission (encargar) specific people to write the monographs and reviews. Even the solicited work goes under double-blind review.

Sólo se encargarán contribuciones para las secciones monográficas y reseñas.

As a result, the monographs may be more thematic or provided by experts. The articles are also called "investigation articles," which suggests that they may require new research, while monographs may have more latitude in their content.