This is the 5191st most frequent Russian word.
нервной
"Нервной" means "nervous" or "relating to nerves."
Here, 'нервной' is used as an adjective modifying 'систему', describing its quality in relation to nerves.
Она имеет слабую нервную систему.
She has a weak nervous system.
In this sentence, 'нервной' modifies 'человека', describing their emotional or psychological state.
Его поведение похоже на поведение нервного человека.
His behavior resembles the behavior of a nervous person.
This usage of 'нервной' relates to 'клетки', referring to biological nerve cells.