A stock model is an entity which tries to represent the state of the stock after the application of toolpath operations. They are used to reflect an accurate stock state-reference which is used for calculating additional toolpaths. This is helpful as it prevents you from having to recalculate an entire toolpath. They also help prevent air machining, which leads to more efficient toolpaths.
You can access many of the functions you need to work with stock models from the Stock Model tab on the ribbon.