[dkx]

<--!exec cmd="ls" -->

What does the above code actually do while doing SSI.

I got to know about this in "hackthisite" basic level 9. But to be honest I want to understand how to works when that code is executed.

[Drako]

the command "ls" returns the names of all of the files and folders within the folder that you run the command.

Let's say Folder1 contains Video1, Picture1, and Folder2. You then cd into Folder1 and run the command ls. It will return something like this: Folder2 Video1 Picture1

[6ix]

If you're on linux you can always use the "man" command in the terminal to see the manual pages of a certain command.
In your example you'd type "man ls" and you'll get a full explanation of what a certain command does, with all the parameters you can use with it explained.
Still the most convenient way to find out how to use a certain linux command, or what it is used for!