It refers to courses run by community members which teach people how to 'snipe' in airsoft safely and effectively.
It's not something you need to do to use a stock sniper rifle at a game, but usage of high-powered upgraded ones requires skill and attention to player safety.
Playing a true sniper role in game is also much more difficult than you'd think.
Everyone will roll in shortly and tell you all about how expensive it is to make a bolt-action springer run better than an AEG and how frustrating it is to jump right into that style of play and be out-gunned. They are right.
But I'd offer that you should run the gun you like and go for it, but only if you can afford to get an AEG to fall back on or another suitable backup.
Make some airsoft buddies and/ or rent for your first games and see what you really like to use before you invest. PS: have fun!