A banjo bolt is a fastener that holds a banjo together, of course.
Kidding. A banjo bolt is one half of a banjo fitting. Banjo fittings are frequently used to pass pressurized fluid. In the motorcycle world, they’re most often seen in brake systems, but there are lots of other non-motorcycle applications for this type of fluid-transfer fitting.
It’s generally a hollow bolt with a hole on the side. That hole allows fluid transfer into the other part of the fitting, which is also hollow. The banjo bolt passes through the center of the fitting, which also has a hole in it. The banjo fitting has flat surfaces which are normally sealed with a washer, often a crush washer. Those seals allow fluid to pass around the bolt, which means the holes do not need to line up unlike a standard threaded fitting, making installation of flexible lines easy (and in many cases, even possible in the first place.)
You’ll see them on your brake lines and hoses, most likely. Some bikes use them for oil transfer to and from various areas, including the cooler, and older motorcycles used them for fuel fittings on the gas tanks.
Banjo fittings are a neat piece of engineering that’s not new, but I still marvel when I see one, because it makes me think of how clever an idea they really are.