Stress is good in certain situations: if you are being chased by a lion your body doesn't need to worry about hunger, your blood needs to go into your muscles, etc. These are typical signs of the stress response: loss of appetite, libido, tense muscles, loss of sleep. There are a group of molecules that are extremely important in putting your body in stress mode, these are cortisols.
Your brain is capable of putting your body in stress mode at any point, in fact, if you measure cortisol levels on people who are struggling with depression in most cases you see very elevated levels of this hormone. It's literally your brain telling your body they are in danger.
Now, if you are working so much you become stressed then your body will suffer and eventually your body will not be capable of dealing with being on "high alert" for so long that it will quite literally shut down. That is burn out, if your work is making you stressed then your work is becoming poison. If you stop drinking poison then your mind will go back to a normal functioning state and the stress will go down.
Burnout is your subconscious mind highjacking your body bc whatever you’re putting your efforts into isn’t worth it at a baser needs level.