when brazing, to pieces of material are put together and the small space inbetween them is filled by a filler metal or alloy. To make sure that the joining peices are not damaged it is important that the filler metal's liquidus (melting) temperature and that it is lower than those of the components being jointed.
By using capillary action, the whole space between both components is filled, so that the joint is solid. Simplified, capillary action is that the level of fluid in a thin tube is higher than the level in a wider tube, if the tubes are connected.
Brazing has a few advantages compared to other joining techniques such as welding. For example brazing will not melt the joining components wich means that tolerances will be finer, the joint will look good so there will not be a need for subsequent treatment, different types of materials can be joined together, even non-metals and there will be less thermal distortion. Finally, it is easy to automate brazing, making it suitable for massproduction.
Of course, there are also some disadvantages. The most important is probably that a brazed joint is weaker than a welded one. It is also very important that the joint is perfectly clean during the manufacturing process, otherwise it may be weakend. The last important disadvantage is that joint is often in a different colour than the rest of the component.