From Brotato Wiki
Item
Tardigrade 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.
Tardigrade
the Tardigrade provides a shield that protects you against 1 instance of Damage. While you have 1 or more Tardigrades active, protecting you, your Max HP bar will be shown as Purple. You can have more than one Tardigrade, which in turn will protect you from that many instances of Damage.
The Tardigrade is most useful on builds that deal damage at range with a low amount of defenses, often called "Glass Cannon" builds. These builds focus on dealing tons of damage at long range, while ignoring the need for defensive stats. Common Glass cannon builds include Characters such as 
Hunter, 
Ranger, and 
Apprentice.
Self-Damage items such as 
Blood Donation and 
Bloody Hand will break the Tardigrade protection (1 per second). So try to avoid picking those up if you already have Tardigrade. Similarly, 
Sick isn't the best friend of the Tardigrade.
In Endless Mode Tardigrades can become a very powerful tool for runs that attempt to go to very high wave numbers, where it can be your main plan for surviving.