Session Conveners, also called Chairs, are the people in charge of reviewing abstracts submitted for presentation in a session. Generally duties of a session chair include advertising the session to encourage abstract submissions, working with co-conveners to determine which abstracts are given oral or poster presentations and if any are not acceptable for the session. They also are in charge of running the session at the conference which includes introducing the speakers, watching so presenters do not go over time, and facilitating question and answer sessions. For young researchers, this is a great chance to learn about the latest results in your research area, as well as meet many of the people working on topics of interest to you. Its also a great leadership training exercise.
Some conferences especially search for ECR participation - keep an eye out for that!