Stuff…

Screen  is a full-screen window manager that multiplexes a physical terminal between several processes
(typically interactive shells). [1]
….
Everything  you type is sent to the program running in the current window.  The only exception to this
is the one keystroke that is used to initiate a command to the window manager.  By default, each  com-
mand  begins  with  a control-a (abbreviated C-a from now on), and is followed by one other keystroke. [1]

This causes a problem when you want to use an application such as minicom, which, also uses CTRL-A key combination to issue commands.  To get around this when using screen you need to do the following key stroke: CTRL-A a

Then, carry on with the next keys required for your application (such as minicom).

An example of this is that minicom requires CTRL-A z to see the help menu… CTRL-A z within screen detaches screen to run in the background which is obviously no good.

The following table shows the default key bindings:
….
C-a a       (meta)        Send the command character (C-a)  to  window.
See escape command.
[1]

[1] lifted from screen man page