"explanatory text" refers to writing or statements that help to make something clear or easier to understand. It provides additional information, details, or explanations about a particular subject or topic. Full definition
Each slide has a short piece of explanatory text accompanied by a picture. (tes.com)
In the book the works are divided into topical chapters: portraits, landscapes, constructions and abstract painting are provided with explanatory texts. (gerhard-richter.com)
Write informative / explanatory texts in which they [i.e., second graders] introduce a topic, use facts and definitions to develop points and provide a concluding statement. (jonathanpelto.com)