Whereas a drop is when you do not move content to a new system, an active delete is specifically removing it from the current system (removing the record in the CMS or removing the file from the filesystem).
This difference is more important than it may initially appear. Sometimes teams want to start working on their site before the new system is in place — actively deleting underperforming content is an excellent activity before the primary transformation starts. This can also ease an actual migration since less material (perhaps for example outlier content structures that would have needed to be massaged for the migration) needs to be transformed.
Active Delete and Forget is the most aggressive (the most pure delete) since, not only do we actively delete it but we forget it as well: we don't even try to usefully redirect if people try to access its old URL.
Note: the distinction between active delete and drop is mostly relevant when changing systems (either the underlying CMS or other systems such as the publishing interface or templating model). That said, even when staying in the same infrastructure there is a slight difference: an active delete means actually removing from the system (for instance, removing the file from the filesystem) — if a file is just orphaned (the file is still kicking around in the system but it isn't linked to) then you have only dropped it.