If, as a result of an illness or accident, you are only able to work reduced hours or not at all, this means you are partially or fully unable to earn. This is often referred to simply as disability. That means you can no longer work in your previous profession at all. You lose part or all of your income because you can no longer perform your accustomed job.
That doesn’t apply, for example, if you become unemployed. In this case, you have no work at the moment, but you are still in principle able to earn an income. Your unemployment benefit is your temporary replacement income in such cases until you find a new job.
The same applies when your doctor writes you off sick for a certain period due to health reasons. We refer to this as temporary incapacity to work. The doctor’s certificate enables you to continue receiving your agreed contractual salary from your employer.