This is the 18223rd most frequent Russian word.
четкого
Clear, precise.
In this case, 'четкого' describes 'понимания', conveying the requirement for clarity.
Для четкого понимания темы, нужно изучать много.
For a clear understanding of the topic, one needs to study a lot.
The word 'четкого' here modifies 'инструкцию', meaning 'clear' in quality.
Он дал четкого инструкцию.
He gave clear instructions.
The word 'четкого' is used here to describe 'ответа', denoting 'clear' or unequivocal.