What is 'river event' in Elgg 2.3.16?

Hi all!

I am new to Elgg. Just downloaded few days ego and started experimenting with it to build a community site like Facebook. I am doing all these in my own PC, so I have plenty of room for R&D. Bow I need some guidance and help. My first question is, what is 'River event' in Elgg. I am using v2.3.16.