Most screen printing inks are partly transparent, so a red printed straight onto a black shirt comes out dark and dull. An underbase gives the top colors a light, opaque surface to sit on. The press prints the underbase, flash cures it so it can be printed over, then lays the remaining colors on top while the shirt stays in place on the platen.
The underbase needs its own screen, so it adds one to the color count and usually to the price. It also adds thickness: a heavy underbase plus several top colors feels stiffer than a one-color print. Printers keep it lighter by using a finer mesh, shrinking it slightly so it hides inside the top colors, or letting the shirt color show through parts of the design.
DTG and DTF lay down a printed white layer for the same reason. On dark blanks, ask whether your design needs an underbase and whether any colors could print without one. The inks guide and cost guide explain both.