It means fewer people are on the road and driving less, but the ones that are, are more likely to die.

For example:

You drive tipsy 1x/month after a party and commute every day. Your death rate is X.

During corona, you don’t commute, but still drive tipsy 1x/month. Your death rate is way higher. Maybe even 10X.

Crucially, your death rate per month remains unchanged.

PS: part of the effect is also lack of practice. You think your reflexes and skills are the same as when you drove daily. They are rusty.

This part kills a lot of middle age motorcyclists jumping back on the bike after 20 years.