Anki & AnkiConnect
Anki is the most popular flashcard app for spaced repetition. It stores your word cards and also helps you retain your learned word by periodic revision.
Game2Text, Yomichan, Animebook and other learning apps require the AnkiConnect addon to connect to Anki.
To install AnkiConnect, launch Anki and go to Tools > Add-ons > Get Add-ons. Enter the code from the AnkiConnect website and Anki will install the add-on.
At the time of this writing, the code is 2055492159.
MacOS users may need to follow the instructions here and enter 3 system commands to their Terminal.