Stress is an adverse reaction that people experience when faced with excessive pressure. If the factors creating the pressure are not dealt with, that pressure can lead to mental and/or physical ill health (eg depression, nervous breakdown, heart disease).
The important thing to remember is that stress is an organisational problem, not an individual weakness. If you are suffering from work-related stress, your employer has a legal duty to tackle it. There is no stigma attached to asking for help.