Hard science fiction is, let's all be honest with ourselves, the nerdier end of the sci-fi world. That makes talking about hard sci-fi books difficult, because, yes, nerds love sci-fi, but they also love arguing about definitions of things and facts. Not that we are complaining, you will often find us there among the weeds of a pointless argument. Bearing all that in mind… hard science fiction is a sub-genre of sci-fi that focuses on scientific accuracy.
If that feels too broad, then we like to think of it like this: imagine a book where the characters are travelling to a distant galaxy to set up a colony. A hard sci-fi book would be more concerned with the technology used to get there and the environmental issues of their new home, than with why they are going and how the folk involved feel about it.
A popular example would be The Martian by Andy Weir. Hank Green was impressed with the science of it and the portrayal of Mars. And if it's science enough for Hank Green, it's science enough for me.