Hello Alok,
If I am guessing right, you are using RestClient class from
TextMarketer SMS[
^] Library located on GitHub. The first line in the try block creates the RestClient instance the first argument being the user name, second being the password of your TextMarketer account. The last parameter represents the environment to be used. The Sandbox environment is used for simulation/testing purpose while Production is used for actually work. The second line actually initiates the delivery process.
How exactly the RestClient works can be found by looking at the code found at the link mentioned earlier.
Regards,