It will sit quietly in your menu bar, left click on it and your Mac will not go to sleep or dim your screen. Another click and you are back to your defaults.
Remember, when you are not plugged into a power source, your battery will drain, as your Mac will not use its power saving features.
It also has the settings in the screenshot when you right click on the coffee icon; they are self explanatory 🙂
This little guy became a must-have tool for my OS X setup and is highly recommended.
It is free and you can download it here.