Meaning of "Keyboard Focus"

[WeThotUWasAToad]

Hello,

What does "keyboard focus" mean as used in the following paragraph?

GuiControlGet, OutputVar, Focus: Retrieves the control identifier (ClassNN) for the control that currently has keyboard focus. Since the specified GUI window must be active for one of its controls to have focus, OutputVar will be made blank if it is not active. Example usage: GuiControlGet, focused_control, focus.

The quoted paragraph comes from the AHK documentation located here:
https://autohotkey.com/docs/commands/GuiControlGet.htm

Thanks

[just me]

All keyboard input will be send to the control which currently has the keyboard focus (in other words: the active input capable control in the active window).

[trismarck]

So to rephrase it a little: when the system detects keyboard input (i.e. the user has pressed the key on the keyboard), the system posts a message (actually, places the message in the input queue associated with a given thread I guess) to the message queue of the thread whose window currently has keyboard focus. That window is I guess usually a control.
A similar scenario occurs for mouse input - when the system detects mouse input, the system posts a message to the message queue of the thread whose window was under the cursor. It works a little bit differently for dialogs I guess and mouse messages aren't really posted to the message queue, but those are some internal details about how winapi really works under the hood.
And for other types of input, Windows I guess takes a similar approach - when the system detects X input, the system posts a message to the message queue of the thread whose window Y.