A Raft is a horizontal latticework of filament that is located underneath your part. Rafts are primarily used with ABS to help with bed adhesion. Rafts are also used to help stabilize models with small footprints, or to create a strong foundation on which to build the upper layers of your part. If your only concern is bed adhesion, you may want to skip to the brim section, as this can typically accomplish the same goal as a raft and is much faster to print.