There is limited research on second language (L2) vocabulary teaching and learning which provides fine-grained descriptions of how vocabulary explanations (VE) are interactionally managed in beginning-level L2 classrooms where learners have a limited L2 repertoire, and how the VEs could contribute to the learners' conceptual understanding of the meaning(s) of the target vocabulary items (VIs). To address these research gaps, we used a corpus of classroom video data from a beginning-level adult ESOL classroom in the United States and applied Conversation Analysis to examine how the class teacher employs various gestural and linguistic resources to construct L2 VEs. We also conducted a 4-month microgenetic analysis to document qualitative changes in learners' understanding of the meaning of specific L2 VIs which were previously explained by the teacher. Findings revealed that the learners' use of gestures allows for an externalisation of thinking processes providing visible output for inspection by the teacher and peers. These findings can inform educators' understanding about L2 vocabulary development as a gradual process of controlling the right gestural and linguistic resources for appropriate communicative purposes.