Tzatziki (Condiment) is the same yogurt-cucumber-garlic sauce considered specifically as a working component on and inside a wrap, rather than as a standalone dish. The angle here is function: what the sauce has to do once it is smeared on warm pita or layered against hot meat. Its job is moisture, acidity, and a cooling contrast to fat and char. It is the binder that lets a handful of loose ingredients hold together as something you can eat from one end without it falling apart. In street-food terms it is structural, not decorative.
In the build, tzatziki is applied to the bread before the meat in most setups, a spread that coats the inside of the pita so the first bite is already seasoned and the bread is partly waterproofed against juices. A second spoonful often goes over the meat and onions before the wrap is rolled, so the sauce is distributed top and bottom rather than pooled in one corner. The texture matters more in this role than in a serving bowl: too thin and it soaks the bread to mush and leaks down your wrist; too thick and it sits in a stubborn stripe that never coats the rest. Done well, it spreads in an even film, stays put through the fold, and keeps every bite balanced. Done badly, it is a wet patch at one end and bare dry bread at the other, or it has been thinned with extra yogurt to stretch a tub and tastes of nothing.
How it shifts depends on what it has to balance. Against fatty pork gyros a sharper, more acidic, more garlic-heavy mix earns its place. Against leaner chicken souvlaki a milder, creamier version keeps the wrap from going flat. Some shops swap in a thinner garlic-yogurt drizzle for speed at volume, which sacrifices the binding job for pour speed. The sauce as a finished mezze is a different thing entirely and deserves its own article rather than being crowded in here. As a condiment, the measure is simple: it should disappear into the wrap as the thing that makes everything else cohere, never announce itself as a sauce sitting on top.