Unknown periods show up when the tachograph can't tell what a driver was doing during a certain stretch of time.
Instead of being marked as driving, work, availability, or rest, that time is simply left unclassified. This usually isn't something a driver does on purpose.
It's a gap in the data, and it can happen for a few reasons.
Common causes
The driver's card wasn't inserted even though the vehicle was being used.
The card was removed briefly during a trip, for example during a ferry crossing or border check.
There was a technical issue with the tachograph or the card.
The vehicle moved with no card in the reader at all, which is still recorded as activity, just without a driver attached to it.