in emacs org mode, I saw #+ prefix many times. I think this is like option but I don't know what it is. I have tried to know them, I have been googling, but I couldn't find any good document. anybody know these symbol? is this org macros?
It is just plain text that you write in an org mode file. It may have two different functions:
1.- If you write it at the beginning of the file, it serves as a metadata descriptor or to include options for the file [1]. For example:
#+TITLE: This is the title of this file
#+AUTHOR: Loretta
#+DATE: 2022-10-15
#+STARTUP: content <--- option for displaying the contents
#+OPTIONS: H:3 toc:nil \n:nil @:t ::t |:t <--- different export options
2.- To create a block inside the file. This block may be code meant to be executed [2], a quotation, a comment, etc:
#+begin_src emacs-lisp
(+ 1 2 3 4 5 6 7)
#+end_src
#+begin_comment
Check the following paragraph. I am not really sure if it
has taken into account all the variables.
#+end_comment
[1] See https://orgmode.org/manual/In_002dbuffer-Settings.html for buffer settings and https://orgmode.org/manual/Export-Settings.html for export settings.
[2] See https://orgmode.org/manual/Literal-Examples.html for code blocks.