A deferred operation is done in your sandbox only until you submit it to the Integrity Server.
For a Deferred Drop, the member working file is removed from your sandbox but the member will not be dropped for any other users. When you submit the deferred drop, then the member is then removed from the project. There are a number of other operations that can also be deferred: check in, rename, move, add, update revision.
To turn on deferred for a specific command, you can set this in your preferences for the command.
To turned deferred operations on globally for all users, set "Deferred Operations Mandatory" in the configuration management policies.