MUNDEX (MUltimodal UNderstanding of EXplanations) is a multimodal corpus for studying the understanding of explanations. It was collected in the Project A02 “Monitoring the Understanding of Explanations” of the TRR 318 Constructing Explainability. The corpus has been build to examine the dynamics of multimodal interaction within ongoing explanations. In the explanation scenario, a speaker (the explainer, EX) explains how to play a board game to a recipient (the explainee, EE). In order to study individual variation in the use of multimodal feedback while avoiding confounding factors such as the explainer's individual explanation strategies, one explainer engages in explanatory dialogues with two or three different explainees one after another. This data publication only contains the annotations of interactions. Video and audio files have not been released in order to protect the participants' privacy. The annotation manual can be found in Lazarov, Türk et al. (2025, https://doi.org/10.17605/OSF.IO/J2DHA).