I think stress in reptiles is produced by conditions that are less than ideal.
examples:
incorrect temps
too cramped
dirty cage
not a good hiding place
sharing a cage with another animal (not always, but it can)
general fear of confinement
etc etc
When snakes are stressed, they may refuse to eat, they may regurgitate, they may bite, they may rub their nose raw trying to escape their cage, strike at the glass ...etc
Excess stress causes reduces immunity in their bodies to ward off disease and infection. Snakes that may have been doing fine with a certain amount of parasites in their system may get much worse and even die with stress as a factor.
In the wild, snakes can choose to do what they need to in order to satisfy their needs. In captivity, we need to know what their needs are and take care of them.
Failing to do so creates a very stressed out animal that knows it cannot satisfy it's most basic needs but can do nothing about it.