Amba (אמבה) is the tangy, fenugreek-driven pickled mango sauce that functions, in a sandwich, as a seasoning strong enough to set the terms for everything else in the bread. The angle is leverage: amba is not a side condiment you can add or skip without consequence, it is a concentrated hit of sour, savory, and bitter that reorganizes the balance of a build the moment it goes in, so the rest of the sandwich is assembled to absorb and answer it. Put another way, the sandwich accommodates the amba rather than the amba garnishing the sandwich.
What gives it that force is its makeup. Unripe mango is salted, softened, and worked with ground fenugreek, turmeric, garlic, mustard, and chili into a loose ochre paste whose dominant note is sour with a warm, slightly bitter fenugreek backbone and a low hum of heat. In a sandwich it is applied in restraint, drizzled or spooned in a thin line, because the flavor is dense and a heavy hand flattens everything around it into the same tangy register. It pairs naturally with fatty and starchy components that can take a sharp counterweight: fried eggplant, soft egg, hummus, grilled meat, fried fish. Done right, amba reads as bright and savory with the fenugreek clearly there but rounded, threaded through the filling rather than sitting in a stripe. Done wrong, it is chalky and raw-tasting, or poured so heavily that the sandwich becomes a single sour note with the rest of the ingredients reduced to texture. The bread underneath, typically a fresh, pliable pita, has to be sturdy enough that the sauce and the tahini it usually rides with do not collapse it.
Its range is wide. The same paste appears in sabich, in shawarma and falafel wraps, with grilled or fried fish, and across a plain hummus plate for those who want the edge. It varies chiefly by sharpness and grind: thin and bracingly sour at one end, thick, mellow, and heavily spiced at the other, with the fenugreek and chili dialed up or down by the cook. Each of those treatments is a recognizable form on its own and deserves its own proper treatment rather than a footnote here, but they all return to the same idea: one pickled-mango sauce assertive enough that a sandwich is built to carry it, not simply finished with it.