The plugin adds a table with metadata (called prefix + termmeta) to use with its features.
The plugin allows multiple parentage for terms, that is, any term can be children (or parent) of any number of other terms. Right now, the only limit is that parent and children should be of the same taxonomy, but not even that is mandatory.
It has useful features for custom templates. On the one hand, you can assign any template to a term, overriding the template hierarchy (actually, hooking into it). But it has also features on single post templates, too: you can assign a template to the posts that are assigned to a given term. Even more, you can assign single post templates on a per-post basis.
Also, you can link any term to a Page. This means that the Page (and its permalink) will be used instead of the template. It also can create automatically the Page and link it to the term for you. You can have the richest term descriptions and prettiest term permalinks ever, by unleashing "the untampered potential of WordPress", the Pages!
Most surely, the term groups are the most powerful and interesting feature bundled with ZYX Term Meta. In some aspects, they resemble taxonomies, but overally speaking they are more powerful and flexible. You can assign any term to one or more term groups; term group data is stored in a global variable (just like $wp_taxonomies); they can have templates, but they can share the templates, too (you can assign the same template to more than a term group); they can be linked to terms, so the term acts as index of a term group; term groups are hierarchical, which means that they can be the parent or children of any number of other term groups; they do have metaboxes, the term group checktables, which are hookable, and display the terms of a given term group (and, optionally, those of the children term groups); checktables also can show the thumbnail, instead of the term name; and term names also may have some checktable-specific features (such as not displaying for a given term group checktable, that of the thumbnail, or displaying a special name and/or hovering description specifically for the checktable).
As implied by the checktable features above, the terms may have term thumbnails. Just select which one of your uploaded images do you want with the dropdown box. The thumbnail has an API resemblant to that of post thumbnails, and the included widgets and template tags already care of thumbnails; so, you can, say, create an "icon cloud" instead of a tag cloud, or display the terms of a given post as icons.