Small items (e.g. glass beads) may be used as markers and placed on top of a player’s own library or graveyard as a reminder for in-game effects. These markers may not disguise the number of cards remaining in that zone nor completely obscure any card.
Players will often put something on top of their deck in order to remind them of an upkeep trigger, such as rebound, and occasionally players will put something on top of their graveyard in order to remind them of some continuous effect. “Small” is very subjective, and there are stories of a player using a plastic fishbowl to completely cover their own deck to prevent them from drawing a card before remembering the upkeep trigger. The key rules are that the item can’t obscure the cards, can’t make it difficult to see any revealed cards, and can’t be a card itself.