Merch is a slang designation for different kinds of products with a certain symbolism. The English word merchandise means "goods, products," and merch is its shortened, slang variant.
Today, when the music industry was significantly blown away and switched to digital format, selling all sorts of merchandise - T-shirts, caps, badges and sweatshirts with the company logo - was not the most recent income for musicians.
An example of use: the famous rapper Oksimiron in the song "City under the sole" is the line "the van is full of boxes with measure".
Merch is in the list: Slang