Basically it's a way of winning DEC if you rank high enough and it doesn't waste your Dark Energy Capture Rate. There is usually a fee to enter and it can be found in the "Events" tab of Splinterlands.
It's a bit time consuming but there are 24 hour rounds so you have an entire day to put in your teams. Sometimes I don't think it worth the effort but sometimes I just like the challenge. Usually if you do well, there's a second round for the top 30% to find the true winner.