This is the 10243rd most frequent Polish word.
błyskawiczną
"Lightning-fast" or "very swift."
In this context, "błyskawiczną" is used as an adjective to describe the swift nature of the reaction, emphasizing speed and urgency.
Wykonała błyskawiczną reakcję, gdy zobaczyła nadjeżdżające auto.
She performed a lightning-fast reaction when she saw an approaching car.
The term "błyskawiczną" in this sentence denotes the characteristic of the delivery service being very fast, which directly modifies the noun "dostawę," meaning "delivery."
Zamówiliśmy błyskawiczną dostawę, aby otrzymać paczkę jeszcze dzisiaj.
We ordered a lightning-fast delivery to receive the package today.
Here, "błyskawiczną" modifies "metodę," meaning "method," indicating that this language-learning method is designed to be executed very quickly and efficiently.