From Brotato Wiki
Item
Padding is an in-game item that can be bought from a Shop or can be found in a Crate. Most Items modify the Stats of the player, and some of them provide additional effects.
Padding
Paddings gives +3 to your Max HP and a further increasing depending on your current carried materials. You gain +1 Max HP per 100 materials you have. This is updated dynamically, meaning that your Max HP will increase during a wave as your pick up more materials. But this Max HP is also lost when you spend your materials in the Shop
The Temperary Materials based Max HP from Padding doesn't work with 
Chunky's Damage increase from max HP. However it does work with the Max HP scaling of 
Chopper, 
Potato Thrower, and 
Alien Eyes.
Padding can be powerful when combined with strategies of saving up large amounts of materials, such as with 
Piggy Bank or with the Character
Saver.
Padding is not a limited item, and you can have any number of them. Multiple Paddings can give you very high amounts of Max HP.