Flashcards can be a powerful learning tool, but they can also be a waste of time.
Flashcards are about retrieval practice. You put a question on one side and the answer on the back. By trying to say the answer before you look, you strengthen the memory link.
Spaced repetition systems (SRS) enhance this technique by automatically scheduling reviews.
Traps you can fall into with flashcards.
Flashcards are vastly superior to passive review techniques. They work well when used appropriately.
If you have used flashcards before, but they were not effective, chances are you either designed the cards badly or other forms of practice may work better for you. If you love flashcards, go forward with caution. Ask if flashcards are really enhancing your learning.