In most of the Linux world, software is organized in packages, which can form a dependency chain. This is a good thing: Let's consider an example
- You want to install a game
- Unbeknown to you, this game is built using the widely-used SDL toolkit
If the system were to naivly install the game but not the SDL runtime, the game would install fine, but it would not run. But since the package containing the game declares a dependency on the package containing the needed SDL components, an installation of the game will automatically also install these SDL components, making the game run fine.
In your case, Gimp allows for scanning of images directly from the application, and it allows for application of effects, that are rendered by ImageMagick. So the Gimp package declares, that in order to become fully functional, it needs these two packages to be installed as well. The package manager will resolve these dependencies and do what is needed.