Sims need to take care of themselves both physically and mentally, otherwise they may run into life threatening consequences.
To learn more about the Main features of Life Improvements take a look at LI General, for information about modules for LI, take a look at the various Module pages.
Sims have to watch their physical health, they can't just sit on the couch all their life!
There are a few ways to increase Physical Health
When Sims reach zero percent Physical Health, they have a heart attack and die.
Sims have to watch their mental health, without stimulation they'll go mad!
There are a few ways to increase Mental Health
Many things factor into how fast or slow Mental Health goes down.
When Sims reach zero percent Mental Health, they go insane. They start talking to objects, talking to themselves, and generally behaving the same way Erratic Sims do.