The current health bar used since 2015.
Health is a property of Humanoid objects that determines the current vitality of that Humanoid. Whenever Health drops to 0, the Humanoid dies, all joints in the model that houses the humanoid breaks, and the Died event fires. In game, when Health drops to 0 in any way except falling from a baseplate, the humanoid will make an "oomph" sound. When a Humanoid being thrown at high speed then the Health drops to 0, the death sound becomes more low-pitched or high-pitched depending on how fast the humanoid is being thrown.
Once a player dies (the player's body parts breaking apart), they must wait 5 seconds to respawn. There is an occasional glitch where the player's body parts do not break apart upon losing all of his/her health.
A script is added by default that causes the health of player characters to regenerate automatically. By default it restores 1% of the maximum health every second. By using scripts, users can alter how fast the health is restored, if it regenerates at all, or even make the health gradually reduce over time.
Health GUI
ROBLOX's health GUI was a bar representing a player's character's current Health (the size of the green or red bar) and MaxHealth (the size of the current health bar is determined in the proportion of MaxHealth). The Health GUI is placed near the center of the bottom of the Player's screen. The standard health GUI can be edited and even removed by Scripts and LocalScripts.
As of 2015 the Health GUI is no longer a separate GUI but is integrated into the default ROBLOX HUD.
History
Since the beginning of ROBLOX, ROBLOX had a hard-coded GUI for the health bar, as in one which could not be modified by the user. Then, as of the introduction to the Lua GUI, the current health GUI now appears instead. At first, the health bar was on the right side of the screen, which is a green bar that turns red when health lowers. Also, blue text saying "Health" is under it. In the 2nd version, it was made bigger. In the 3rd version, it changed completely, it is now on the bottom and white text saying "Health" is inside it. Also, when a large amount of health is reduced, the health bar flashes red, when the player's health was low, the health bar turned red. A notable change is what looks like blood when the player dies instantly or loses a large amount of health appears on the corners of the screen. On the 4th version, the health bar no longer blinks when a large amount of health is lost, also, the text font changed.
The health bar used between 2014 and 2015.
On August 27, 2014, the health bar was simplified with a lighter green color and with rounded edges. The word "Health" was removed, and the bar will now look white when a player loses all of their health.
In 2015, the health bar was put in the leaderboard, below the player's name, with square edges and is thinner than before. In 2017 a change has been made so that no health bars will appear when the health is equal to the maximum health of a Humanoid, this includes the default health bar that shows your health and the health bar that appears above humanoids.