How do you say flash cards in spanish?

The concept of "flash cards" i.e. index cards for memorizing material, not a glittery piece of paper, does not exist in Spanish. The best way to translate it would be to translate the concept: "Tarjetas de memorización" - "Cards for memorizing"

"Tarjetas de destello" are cards that are flashy or bright (like the glittery piece of paper).