on the coin is fully hidden in the tread, your tyres are above the legal limit and you can go about your day. if you can see any of the band however, it's likely that your tyres could be unsafe and require professional inspection by a mechanic and maybe replacement.
when you are driving such a huge piece of machinery around, and responsible for other people, it is always important to have these basics checked regularly, for your own safety, as well as everyone else. 