These rules apply to people who are claiming benefit on the basis of illness (Incapacity Benefit, Income Support or Severe Disablement Allowance). It is possible to do some paid work and not lose the benefit but there are time limits for most people on how long they can do this for.
The nature of the permitted work has to be considered carefully and it is important to get advice before you start to use these rules about how it will affect your benefit. There are earnings rules for how much you can earn. The amount you can keep if you claim Income Support because of incapacity is quite low.