This is the 17361st most frequent Russian word.
твердость
Firmness or hardness.
In this sentence, 'твердость' is used in a physical context to talk about the material property of steel, referring to its hardness.
Твердость стали определяет её прочность.
The hardness of steel determines its strength.
Here, 'твердость' describes an abstract quality of a person, reflecting resoluteness or determination.
Твердость его характера помогла ему преодолеть трудности.
The firmness of his character helped him overcome difficulties.
This usage of 'твердость' again pertains to its technical meaning as a physical attribute being tested or measured.