All the stuff that is carried in a bike bag to repair a mountain bike is of little use if no one knows how to use them.
The fact is someday, sometime the bike a rider is riding will break in the middle of nowhere. The alternative is to fix it or walk. Fixing it is usually easier.
Here are the things a rider should know how to fix. It is suggested some practice is done at home so that the lesson doesn't have to be learned on a 100 degree day in a place where there is no shade.
Fixing a flat tire. Removing the wheel from the bike, peeling off the tire with tire irons, using the patch kit or replacing with another tube without punching another hole in it and reassembly.
Derailed and broken chain. Understand the tensioning and how the chain breaks down if need be. Be prepared to deal with grease (carry a rag).
Loose or broken spoke. A bad spoke can be a real irritant especially when riding a difficult part of a trail. Know how to loosen, replace and retention properly.
As for maintenance, some may prefer to have a shop do all their major maintenance, but there are certain maintenance tasks that need to be done often. Riders should learn how to do these themselves. These include how to:
Clean the chain.
Lubricate the chain and wipe off excess lubricant.
Check and adjust tire pressure.
Check wheels for alignment.
Clean the bike and keep it that way.
Lubricate cables and derailleurs.
Most of these need to be done after just one good ride.