It’s short for “merchandise,” and you know how the kids today like to abbrvt evrthng.
When you purch merch (“purchase” “merchandise”), it goes to a great cause—’cause I need the money. But it also tells the boys in marketing which products and revenue streams the masses prefer; it’s kinda theoretical, so just remember that if you want to support the arts, buy something. It’s like leaving a big tip at a restaurant: you’ll feel good about yourself and the server will be happy. Especially since they didn’t spit in your food.