Tzatziki is a creamy, tangy Greek sauce or dip made from strained yogurt, grated cucumber, garlic, olive oil, and herbs like dill or mint, often with a splash of vinegar or lemon juice. It's served as a meze (appetizer) with pita bread and vegetables, or as a condiment for grilled meats like souvlaki and gyros, adding a cool, refreshing element to many dishes.