The isGrounded is not a command, but rather a bool, meaning it will be either TRUE or FALSE. In the code you provided, the Update method (runs every frame) is checking to see if the controller isGrounded with this line here:
if (controller.isGrounded)
The above code is the same as this code below, which is sometimes easier for new programmers to understand.
if (controller.isGrounded == true)
What the script you provided is doing is only accepting input to rotate or add force to the character controller based off the keys W, S, A, and D, while the controller is grounded. If you wanted to do something when the isGround bool is false, you would do one of these:
if (!controller.isGrounded)
if (controller.isGrounded == false)
As far as the frustration goes, don’t worry, that is normal. Take a break, do something fun, and come back recharged. Being new to programming, I would highly recommend looking into some learning material, such as Unity’s own scripting tutorials, which are fantastic for new Unity developers.