In the context of 3D printing, a raft is disposable material on which the object is printed instead of being printed directly on the build plate. Often times a raft is used to prevent warping or to help the print stick to the build plate.
The 3D printer will first build the raft and then the object on top of the raft.
Consumer 3D printers will usually build the raft out of the same material the printed object is made of; for example, ABS or PLA plastic.
Commercial printers often build the raft and supports out of a different material than the object. This makes it easier to remove the raft when the print is complete.
Some printers have the option to print with or without a raft whereas other printers require a raft.