The Project deadline indicates the date by which Suppliers must submit their response to your project. After the deadline expires Respondents may no longer edit their answers or submit their response. You do not need to specify a deadline, and you can always modify the date later.
Note that there is also a Deadline field associated with each Response. By default SupplierSelect will keep the Project and Response Deadlines synchronised. However, it is possible to change the Deadline for one individual Response.