Survival Essentials adds accessibility to a bunch of features which can improve your survival gameplay. It’s like a cheat menu specifically designed for survival. For example, give yourself food items, a diamond kit or toggle fly mode on and off. You can of course use it in creative mode but some features, like giving yourself food, is obviously less useful in creative mode.
Press the ESS button on the right side of the screen to open the survival essentials menu. These are cheats which you can use to improve your survival gameplay. Some of the cheats include flying, armor and weapon kits, food items and enchanting.
You should of course only use this in single player (I’m not even sure if it works in multiplayer). It’s really useful if you are a beginner (or awful survival skills) and need to improve your chances of survival. It’s also useful if you want to be able to access certain features, e.g. flying, while in survival mode.