This is the 367th most frequent Polish word.
mocno
"Strongly" or "firmly."
The word 'mocno' here emphasizes the manner in which the action of holding took place—tightly or firmly.
Ona mocno trzymała jego rękę.
She was holding his hand tightly.
In this context, 'mocno' conveys the intensity of the belief—strongly or deeply.
Mocno wierzę w sukces tego projektu.
I strongly believe in the success of this project.
Here, 'mocno' modifies the verb 'zderzył się' to explain the force of the impact—heavily or hard.