Fortunately Wikipedia comes to my aid, but basically Amber Diceless is a roleplaying system based in the cosmos of Zelazny's Amber serieses. The key difference that it has with other systems is that no dice are used; decisions are made by storyteller fiat, aided by the player's descriptions and the character's statistics.
It is the first system in which I played in a campaign.
"All roads lead to Amber," he said, as though it were an axiom.
--Corwin, quoting Random, Nine Princes in Amber