A macro is a prerecorded chat command accessed via a key on your hotkey bar.
A macro is created using the following command:
/macro nnn /n text
"nnn" is a 3 letter title you want to give this macro. "LFS" is commonly used for a broadcast macro used in advertising for a squad invite.
"/n" is the channel you want the macro to appear in, such as /b (broadcast), /l (local), /o (outfit), or (in very rare cases) /c (command chat).
"text" is the text you want to have appear.
Example: /macro LFS /b BR17 CR2 with adv hack and air cav LFS
Hitting the key linked to the macro "LFS" in your Hotkey Bar will then put "BR17 CR2 with adv hack and air cav LFS" into your local broadcast channel.
You can also add Color Codes to your macro to add that extra colorful flare to it.