This is the 6438th most frequent Russian word.
электронная
Electronic.
In this sentence, 'электронная' (electronic) modifies 'почта' (mail) to specify the kind of mail being referred to, indicating its digital nature.
Электронная почта стала основным средством общения.
Electronic mail has become the main communication means.
Here, 'электронная' modifies 'книга' (book), describing it as a digital device or format rather than a physical book.
Я купил новую электронную книгу для чтения.
I bought a new electronic book for reading.
The word 'электронной' modifies 'картографии' (cartography), referring to mapping methods that utilize digital or electronic technology.