The Stat Bar
On your stat bar you have three stats that you have to worry about. Health, Stamina, And your Heat.
Your Health represents how alive your character is. When this is reduced to zero your character is knocked out and medics will come to bring you to the infirmary, taking any contraband you were carrying with you at the time. Health very slowly regens on its own but regens much faster by resting in a bed. You can also use some consumables to restore health. There is no way to increase your health but wearing armor makes you take less health damage with each hit making you harder to knock out.
Your Stamina represents how much energy your character has. When this is reduced to zero your character will be unable to do most actions aside from walking, talking, and picking up / dropping items. Like Health Stamina restores on its own but can be sped up by doing actions such as taking a shower or lying down. And also like health many consumables restore stamina. It is a good idea to bring consumables if your going to be doing a lot of work during the night as lying in your bed will speed up the night making it hard to restore stamina (unless your playing in multiplayer in which the night will only speed up if everyone lies in there bed at the same time).
Your Heat represents how much negative attention you have from the guards. Unlike the other two stats your generally going to want to have this number as low as possible. Doing actions like being late for a routine, starting a fight with an inmate, or being in another inmates cell will raise your heat. Once your heat reaches about 80 most guards will attack you on sight, and at 100 being outside will result in being shot by snipers. Actions that represent active escape attempts or aggresion to guards will often result in an instant 100 heat. Unlike the other two stats your heat will gradually decrease over time and there is no real way to speed this up.