Believe it or not the answer is about as short as the question. Brazing is the process of connecting two or more metals together with a filler metal and heat. Brazing temperatures are 840°F (450°C) and higher. During the brazing process the metals are connected at a point called a joint using a heat source like a torch or oven. In other words, this joint holds the two metals together using an alloy (filler metal) with a lower melting point than the two metals. If you think about it, this is the similar to a grilled cheese sandwich. The bread is the two metals, the cheese is the alloy, and the heat is what keeps them together. Without the heat there is little to no contact to keep the two pieces together.
When heating the filler metal above it’s melting point in an furnace, it needs to protected form oxidation by a suitable atmosphere. The atmospheres in which the brazing process can be undertaken include air, disassociated ammonia, nitrogen, hydrogen, and vacuum. If a torch or non-atmospheric oven is used as the heat source, then a brazing flux is required.
The molten filler metal cools to join the work pieces together providing a strong join between similar or dissimilar metals. A sound brazed joint requires the filler and base materials to be metallurgically compatible. Also the joint should have a gap for the molten filler metal to flow into.