In PIC Basic Pro compiler DEBUG is one of several built-in asynchronous serial functions. It is the smallest of the software generated serial routines. It can be used to send debugging information (variables, program position markers, etc.) to a terminal program like Hyperterm. It can also be used anytime serial output is desired on a fixed pin at a fixed baud rate.
In very simple words, DEBUG is an asynchronous serial output to fixed pin and baud with mode 0 denotig True while mode 1 denoting Inverted. So in your case, you dont use inverted TTL.