From Toadwater Guide
The top bar in your Info Center represents how much health (known as hit points in other games) you have. Most actions cost some health.
No one can kill you but if your health drops below zero from your own actions, you will earn a Death. Familiarize yourself with the different actions and how much health they cost. You can select the ranges in which you would like your health bar to be green, yellow, and red by going to Game>Options.
You regain health over time, up to your personal maximum, which is normally equal to your Level (which is determined by your overall Experience). If you eat a Gold radish your maximum will be increased by 10%.
There are various other items that will increase your health: see food for details.