So, there are a number of functions that are called continuously while you're game is running. As mentioned in the docs you linked, those are
As explained there, the
_process function is called once per frame while your application is running. Since frame rates may vary (sometimes wildly) during a game (or between different games), the frequency of the calls to
_process may vary somewhat from frame to frame.
delta argument that's passed in to
process is just the amount of time since the last time
_process was called (in the previous frame). For a game running at 60fps (for example), the
delta value will be approximately 1/60 of a second. But, remember, frame rates vary based on lots of things. So, on one frame it could be 1/60 or 1/55, or 1/30, or ...
That value is important when you're trying to make something appear to operate consistently, regardless of varying frame rates. That
delta value can be used to help "scale" motion (for example) to keep the look / feel consistent. For example, the distance that a running character needs to move in one frame will be different for a frame that took only 1/60 as compared to one that took, say 1/30 of a second. Again, that value allows things to be "scaled" according to the duration of the frame in an effort to make then appear consistent to a player.
_physics_process function is similar, except that it fires at a consistent rate - based on your project's
Physics Fps setting (default = 60 fps).
Back to the high-level overview though, when a Godot game is running, all of the
_physics_process functions found in any script will be executed once per frame or physics frame as outlined above.
So, these are good places to put things that must constantly be monitored, such as User input.