Yes. Work Units are serial in nature. When a completed WU is sent back, a new work unit is generated from those results. This must happen many times over within each project (group of work units). A generation 1 work unit must be turned in before a generation 2 work unit is created and sent out.
To keep these generations moving along, we have to set expiration deadlines in the event a work unit is not uploaded in a timely manner (lost, deleted, whatever). These unfinished work units “expire” and are reassigned to new machines. You will still receive credit for all WUs completed and uploaded prior to the Timeout (formerly preferred deadline). However, after the Timeout, your contribution is not as useful scientifically because another copy of that work unit had to be sent out to another contributor. Even if you eventually complete the work unit, that other contributor still had to process duplicate work to assure the science moves forward. And it would be unfair not to also credit that second contributor.
Even so, full credit is given up until the Deadline (formerly Final Deadline). After the Deadline has expired, the client will automatically discard the work unit and download new work. If you have trouble completing work units before the Timeout (formerly Preferred Deadline), it is recommended to either run the FAH client more hours each day, or to run the client on a faster computer.
As we move to larger and longer WUs, we will extend the expiration time as needed. Deadlines vary on the order of a few days to a several weeks, depending on the nature of the WU. Turn in a work unit just before the deadline is not the goal. It is most helpful to the project to return work units as quickly as possible. And how these deadlines are determined is explained a few answers below.