A plenum in language teaching means the whole group, so an activity in plenum / plenary will involve all learners and the teacher. This can be compared to closed pair and group work.

Example
Learners work in pairs to correct the errors in a text, then in plenum work with the teacher and an overhead transparency on a final version.

In the classroom
Learner groupings are an extremely important part of classroom management. Teachers decide what kind of grouping best supports the aims of the activity they are doing. Possible groupings include; closed or open pair, groups of varying sizes, or whole class with or without the teacher.

Further links:
https://www.teachingenglish.org.uk/article/group-work-v-whole-class-activities
https://www.teachingenglish.org.uk/article/getting-whole-class-talking
https://www.teachingenglish.org.uk/article/getting-whole-class-talking-0