On a turn, you play one or two cards onto one or both of the two discard piles. On one pile, you must play a higher card than whatever the top card is, and on the other, you must play a lower card. The exception is that you can play any card on a discard pile if it's the same color as the current top card. Thus, on the ascending discard pile, the cards played might be blue 1, red 2, gray 4, yellow 6, yellow 2 — and that drop-down gives you breathing room in which to play more cards.
You can't reveal the specific numbers you have in hand, but you can say things like, "I have a high yellow card." In the "professional" version of the game, you must play exactly one card each turn (even if you want to play two), and you can reveal only the color of cards in your hand and on which stack you want to play.
