This lets you know that you are looking at a function for the Python scripting language.
A complete list of these functions (with their definitions) is available wherever you can add a script. Just type system. and then press Ctrl+Space to get a list of all the functions available. If you keep typing, the list will even be automatically narrowed down for you!
You can see below there are many different categories of system functions available for your use. For an overview and syntax scripting, refer to the Python Scripting section.