Concurrent sessions are those where you will have multiple presentations (or papers presented) at the same time and date. This is an alternate method for grouping similar sessions at a conference. For example, you can choose to view all presentations that are available in Block C, which is 10 am to Noon on a Wednesday. All the topics may be totally different from each other.
A different use by EventsAir is Tracks, where all topics are categorized by a common theme, or topic. For example, you could choose to view all topics in the Introductory Track or the Advanced Technology Track. All sessions in the same track will be at different times and dates, but you may be more interested in planning your conference attendance by choosing presentations by track.