Bash is a shell and a simple programming shell too. There are a lot variables built in or externally specified. If a variable is specified externally making this variable system wide available is a problem. Export command makes variables available system wide by propagating to the sub shells. Exported variable would not be available to the parent processes. Which means previously spawned or created process will not get exported variable.
Syntax of export is like below there is two way to export some variable.
In this case we export all ready defined variable to the sub shells.
$ export myfw
Export After Defining
In this model variable is just defined before exported. After variable definition variable exported to the system wide.
$ export httpserver=192.168.1.10
List Exported Variables
It can be listed exported varibles with -p parameter.
$ export -p