If something or someone is described as being "hard as nails," it means that they are either physically or mentally tough. The phrase can also be used to refer to someone who is lacking in emotion and isn't concerned about the emotions of others. Someone who is "hard as nails" is often someone who has had a difficult life and, as such, has become numb to suffering and hardships. This phrase is one of many English idioms that have similar meanings and use the word "hard," such as "hard as a rock" or "hard-hearted."
An idiom is a word or phrase which is used as a way of adding color or impact to English speech that might otherwise be dull or mundane. The distinguishing characteristic of an idiom is that its meaning is often accepted to be something far different than what the literal definitions of the words that comprise it might suggest. Its meaning can even differ from what it meant when the phrase was first coined. One such idiom that is popularly used is the phrase "hard as nails."
The most common way in which this phrase is used is as a means of describing toughness. There are certain people who have an ability to endure physical pain much better than normal people might be able to withstand. By the same token, some people can deal with mental or emotional setbacks with the same kind of unwavering attitude. In either case, this phrase can be used. For example, someone might say, "Considering how much pain he must have been suffering, that guy is hard as nails to get that surgery and be up and walking around the next day."
There can also be an implication with this phrase that the person described in this manner can be unfeeling or harsh toward others. The phrase, when used in this manner, seems to diverge from being about the toughness of a person to being about the tough way that person treats other people. This is often the case when someone has led a life full of problems and difficulties. As an example, consider the sentence, "He was brought up by really nasty parents, so it's no wonder that he's as hard as nails."
"Hard as nails" is one of many phrases using the word "hard" to denote toughness. In this case, the meaning is relatively obvious. Nails are generally made of the most durable material possible, so anything or anyone that is considered as hard as that, in a figurative sense, must be particularly tough.