Major medical congresses grant plenary sessions to data that they feel are of critical importance and/or of great interest to their attendees.
The process for submitting to a major meeting is as follows: 1. You write an abstract, usually somewhere in the 300 to 500 word range 2. You submit just the abstract to the meeting. They use that to decide on acceptance for the meeting and whether or not a plenary will be granted 3. You write the poster (and the presentation if accepted) 4. At the meeting, you present the data live in a plenary session, if granted 5. The poster is in a big room with hundreds of other posters. The attendees walk around and read the posters and talk with the authors, who are usually present at their poster. Sometimes they have paper copies of the poster you can grab.
Basically all it is is an oral presentation of the data in the poster. All plenaries are accompanied by a poster.
And the 2-hour timeframe is just when the poster exhibit is open and authors are likely to be near their posters. They often leave the poster room open all day (after the plenaries are finished).