MOV    DPTR,#FREQTBL    ;POINT TO LOOK UP TABLE

what does the statement Mov DPTR, #FREQTBL means.

It does what it says in the comment - it loads the address of (the start of) a lookup table into the DPTR.
The DPTR always has to be set up for the MOVX instructions to work; eg, this 'C' statement:

debugTxD = 0;

could be implemented as follows in assembler:

MOV  DPTR,#debugTxD ; Load the address of the debugTxD variable into the DPTR
CLR  A              ; Set a value of zero in A
MOVX @DPTR,A        ; Write the value from A to the location addressed by the DPTR 
                    ; - ie, set the debugTxD variable to zero!

For further discussion of lookup tables, use the 'Search'