Waiting day(s) is (are) unpaid day(s), first day(s) of incapacity for work. If you have a phase A contract with a temporary employment clause, two waiting days apply. One day of this is immediately compensated with your salary. If you have a secondment agreement (for a defenite or indefenite period), then there is one waiting day.
On waiting days, when renting accommodation via Charlie works, you also have to pay for rented accommodation. Regulations about this subject can be found in the housing regulations.