A word is 16 bits...

16 bits gives you a range from 0-65535

The internal ADC in most PIC's is 10 bits...

10 bits gives you a range from 0-1023

You need a WORD variable if in PBP you want to play with numbers bigger than 255 (a BYTE). When reading from your ADC, your WORD variable will acquire a value in the range 0-1023... the upper bits will be zero.