Sumac is a deep red spice made from the dried, ground berries of a shrub that grows wild around the Mediterranean and in the Middle East, Sicily included.
Its flavour is tangy and slightly astringent, similar to lemon but rounder: in the Middle East it was used to add sourness to dishes before lemons were common. It is one of the ingredients of za'atar and fattoush.
I use it raw, sprinkled at the end: on yogurt, hummus, potato salads, roast vegetables and grains. As well as flavour, it adds a beautiful colour.