Per default, Linux does not allow you to run executables in the present dir (it will pref. one in your PATH) thus we need to tell the OS that we need to run the executables in this dir (thus the . meaning this dir).
When working on the command line (CLI), a period means current directory. If you could print the contents of the period, it would be the same as the output of the 'pwd' command.
If this was valid: 'echo .' (or) 'echo $.' it would have the same output as 'pwd'.
When you type something on the CLI, it looks through your path variable to see if it can find a folder in the path which contains the executable name. If it cannot then it will spout command not found or whatever. If you type:
echo $PATH then you will see all the folders that are searched when looking for an executable program/script. If you would like to add to this (say you have a folder of perl scripts you wrote and you want to access them from any folder) you can do this:
This will wear off when you log out of your shell however. If you would like this new path to be in your shell when you login, then you need to put it in your .bashrc (if you're using bash).
Don't export the period to your path, it could have security implications.
Note - Instead of doing ./configre or whatever you can always type out the whole path.... eg:
(above command is same as)
Note 2 - If (using the example above) you do this: