It is like digging the trenches and filling them back. Maybe even worse.
The defining feature is this: one so completely pointless that even the person who has to perform it every day cannot convince themselves there’s a good reason for them to be doing it. They may not be able to admit this to their co-workers – often, there are very good reasons not to do so – but they are convinced the job is pointless nonetheless.
Bullshit jobs are not just jobs that are useless; typically, there has to be some degree of pretence and fraud involved as well. The employee must feel obliged to pretend that there is, in fact, a good reason their job exists, even if, privately, they find such claims ridiculous.