Khubz b'Zayt (خبز بالزيت) is bread with olive oil, the simplest possible Lebanese open sandwich: flatbread dipped or drizzled with good oil, usually carried further with za'atar. The angle is that with so little on it, the two things on the plate have nowhere to hide. The bread has to be fresh and the oil has to be genuinely good, grassy and a little peppery, because the whole pleasure is the contrast between warm, soft crumb and the fruity bite of the oil soaking into it. This is a snack and a breakfast as much as a sandwich, and it is the baseline against which every more elaborate flatbread build is a variation.
The build could not be shorter, which is exactly why execution shows. A round of khubz, ideally still warm, is opened or left whole; olive oil is poured over it or it is torn and dipped; za'atar, the thyme-sumac-sesame blend, is scattered on so it sticks to the oiled surface. Some make it as a quick fold, oil and za'atar inside split bread pressed together; some lay it open and eat it in torn pieces. A good khubz b'zayt has bread that is soft and pliable, oil that is fruity and well distributed rather than pooled in one spot, and just enough za'atar to season every bite without going dusty and dry. A poor one is stale bread that cracks instead of folding, oil that is flat or rancid, or za'atar piled on so thick it is all you taste. The margin for error is small because there is nothing else in the build to absorb a mistake.
It varies mostly by what gets added to the oil. Za'atar is the near-universal partner and the most common form, but the same idea accepts a rub of fresh thyme, a scatter of plain sumac, sliced tomato and cucumber tucked in, or a knob of butter under the oil so it carries further. Pushed a little, it becomes its cousin on the saj, dough spread with oil and za'atar and baked rather than dressed after the fact, which is a distinct form worth its own treatment. What stays constant is the principle: a good flatbread and good olive oil, brought together while the bread is fresh, with seasoning added but never allowed to bury either one.