It's the parts of a program's concepts, rules and behaviours that are specific to the program's task. For instance
- Items, a shopping cart and the conversion of such a cart into an order at checkout in an e-commerce application.
- Clips of video and audio, static images etc. and the compiling of these into a single output video for a video editor.
- Vertices, triangles, meshes, animation rigs etc. for a 3d editing program.
- Accounting standards and tax laws for an accounting system.
When developing software you deal both with these kinds of specifics and generically reusable concepts that are more purely computational science, so a term to distinguish them is handy.
Fun fact, snakes' adaptations to their feeding style are actually not about the way the jaw hinges. Instead, their lower jaw is two separately moving bones held together with stretchy ligament tissue so that each side of the mouth can be "walked along" the prey item separately.
So the chef could eat the burger... but would follow along its longest dimension to do it, laying it down sideways.