Callbacks generally begin with an underscore. A callback is a function which the engine calls at specific points. For example, _ready() is called once all of a node's children are loaded and ready (or immediately upon being loaded, if it has no children.) Other callbacks include _input(event), _process(delta), and _physics_process(delta)
Functions that are connected to signals are a type of callback, and might have a name like _on_button_pressed()
It can also indicate that a function is meant to be used only within that script, as GDScript has no concept of encapsulation (outside of needing a reference to the node, I suppose.) So if you want to discourage calling _my_private_function(), give it an underscore at the start.
