There are many meanings in the word "stock", but it's hard for me to figure out which one it is in the web dictionary.
The web dictionary: https://www.thefreedictionary.com/stock
And here's the definition of “Stock Media Product” in the document:
“Stock Media Product” is the collection of one or more digital files, packaged in the form of a product on the Site that can be identified by a product ID, and that is uploaded by you, or otherwise made available to you, for Purchase on the Site. A Stock Media Product may include 3D models, texture maps, motion captures, photographs, imagery, application and plug-in software, materials and shaders, shapes and vector graphics, training materials, building components, sound effects, stock music, and videos. Additionally, a Stock Media Product may include a number of individual files of different file formats to make it easier for Customers to work in a variety of software applications (such as 3ds Max, Maya, and Cinema 4D), and may also include other files (such as jpeg images used for texturing), and images or other files that are used for promotional purposes. For Customers and their use in Creations, this definition also includes derivative and intermediary files used for that purpose.