Plein air paintings are done entirely outdoors on location. (Except for maybe using a finger later to soften an edge or two, but don't let them know I said that.
Look up the information about the plein air event. They probably have a website that will answer a lot of your questions. Their website probably has contact info. Call one or two of the organizers with any other questions you might have.
I wrote a few other things on this thread:
Then go out and practice, and go have fun!