Short Answer
A Terminology can be defined as a system of specialized terms relating to a particular subject, or research field.
Detailed Answer
Using terminologies is a way to provide data with a formal concept. Terminologies differ in their complexity. They can reach from simple lists of specific terms to terms that are related by more complex relationships.
In the context of GFBio different formality levels are defined:
Controlled vocabulary: List of terms
Glossary: List of terms with definitions
Taxonomy: Terms related by hierarchical structures
Thesaurus: Extension of the types of relations by, e.g., synonyms
Ontology: Formal specification that allows the modeling of very complex relationship structures and axioms
If data are connected with terminologies they can be searched and accessed with standard routines, for instance within a search engine that enables the search for related terms, synonyms etc.
By using terminologies your published data or metadata can be found and accessed more easily.