This is the 2241st most frequent Russian word.
жесткие
"Тough" or "rigid."
In this sentence, 'жесткие' is used to describe the physical property of the chairs being 'hard'.
Эти жесткие стулья неудобны для долгого сидения.
These hard chairs are uncomfortable for long sitting.
Here, 'жесткие' is employed metaphorically to mean 'strict' when describing rules.
Он вводит жесткие правила для всех сотрудников.
He is implementing strict rules for all employees.
Again, 'жесткие' is used metaphorically to mean 'strict', this time related to measures.