What does 'is very hard' mean?

If something 'is very hard' it means it is solid - for instance if you take mud that is soft, while a slab of granite is very hard in comparison to the mud.
It can also be used to describe a person, someone who is very hard can be a muscular person who is strong and good in a fight or someone who is mentally brutal like a boss who fires staff without feeling for their situation is mentally very hard.